Sorry to necropost this. Okay, I have been a Redditor for around one year and I can tell you, OP, something I learned from the Reddit system.
If multiple accounts mutually share the same IP address (REGARDLESS of posting activity or total karma) and vote on the same post or comment a downvote or upvote (whichever one is opposite) will be added on top of the second account's vote. For example, let's assume a post has 2 points 100% upvoted. If I upvote a post on my main account and I also downvote it on my side account, it would show 3 points 80%. In other words, your main account upvoting would show 3 points 100%. Then if your side account downvotes, it would not show 2 points 75% but instead would show 3 points 80%. Which means that yes the system does add in an extra vote. Now if I were to use another side account and upvote this again on the same IP address, it would show 3 points 71% upvoted.
Apparently, it does not truly matter how much an account has been upvoting or downvoting each other interactively. Honestly, I tend to use my main account at home IP address while at work I use my side account. I shit-comment/post all the time on my side account and then when I come home, I upvote any comments/posts that my side account got downvoted on. And guess what? They all gained karma 1:1 ratio (except for text submissions lol). I did this vice-versa and it worked fine.
I've done this for around 6 months daily mostly, and the mods still haven't caught me. It may be the way how frequently I post comments on both accounts (emulate 2 different people). I'll try to test something with another dummy that votes both my accounts but use another unique IP address.
Also, I believe the amount of total comment karma DOES influence whether a vote counts or not. I sometimes notice a downvote from a person with a low karma score (idk lower than 30?) that downvote me yet I still didn't lose karma. This shows that new accounts also have a probation of their vote counting or not. I think this happened to my alt account as well in my early days.
Now I'm not sure if there are any Reddit admins that are looking after me but I have not been touched with a single shadowban on either account. Despite many times where my alt and my main shared votes on the same IP address login, we still didn't get hit or warned by a ban.
TLDR: if your vote counts, then you will see a karma gain or loss.