Why does my stitch count keep drifting on amigurumi rounds?

That stitch drift with increases is super common and yes, there's a real trick for it. When you do two stitches into the same spot, the second stitch goes into exactly the same hole as the first one. Not the next hole over, not the space beside it. The same hole. What usally happens is people work the increase, then the next stitch naturally drifts into the following stitch instead, so you lose one somewhere and your count is off by the end of the round without anything looking wrong as you go.

Two things fixed this for me. First, after every increase I'd physically poke my hook into the two stitches of the increase to confirm they share a hole, takes two seconds. Second, I stopped relying on the marker alone and started counting the whole round out loud after finishing it, every round, before moving on. Annoying? Absolutely. But catching an error at round 6 takes thirty seconds to fix with a bit of frogging, versus discovering it at round 12 when the leg looks like a shoe.

On your counting question: an increase counts as two stitches. So if a round says 'increase across' and you end with 24 stitches, that's 24 total, increases included as two each. If your pattern says something like 'sc 1, inc' around, the increase itself is two of those stitches. Most counting errors come from people counting the increase as one, then wondering why they're short. For the two-stitches-in-one-spot issue specifically, some people find it helps to use a running stitch marker, a bit of scrap yarn you just carry up the side and move at the end of every round, so you can see exactly where the round starts and ends without moving a plastic marker each time.

And honestly, the wonkiness settles down once your counts are consistent. Symmetrical arms come from identical tension and identical stitch counts on both sides, so once the increases are landing in the right holes, it sort of fixes itself.

Honestly the thing that saved my sanity with tiny-count amigurumi rounds was one of those clicker counters that clips onto the hook. I used to lose my place mid-round every time my kid yelled something from another room, and one click per stitch meant I always knew if the count was short before I hit the end. Cheap, dumb, worked like magic for me. Also on the 'eyes say right, math says wrong' thing, I get it completely. My fix was to learn what an increase actualy looks like when it's done, because once I coud spot that little leaning pair of stitches, I could find my own errors without counting the whole round back.

One habit that helped me more than any marker was learning to count the little V-shaped loops across the top of the previous round instead of recounting my own stitches as I go. Every single crochet leaves exactly one visible loop per stitch, so before starting a new round I run my hook along and count the Vs, which catches a drift before I've built on top of it. Pedantic, yes, but it turned my error rate around completley. Also worth checking wheather your frog pattern is worked in a spiral or with a slip stitch join each round, beacuse joined rounds shift where the round actually begins and can make a perfectly correct count look wrong at the seam.

Mine was a wonky octopus with one tentacle longer than the rest and I swear he's still my favorite of everything I've made. The wonk kind of grows on you.

One thing that helped me with matching legs was making them back to back and holding them up against each other every couple rounds instead of finishing one and starting the other. If they're already drifting you catch it while there's still time to fix it, and you don't end up with a pair that only matches in theory. Also don't stress too much about tension early on, mine evened out on its own after a few projects without me doing anything different.

Update: started counting the little V loops across the top of the previous round before starting each new one, like one of you suggested, and that alone fixed most of my drift. Legs actually match now. Still stuck on the increase question though, when a round says increase across, does that count as two stitches at the end-of-round check or one?