Why is one screw in V-brake a lot tighter? I noticed that whenever tuning my V-brakes, I always have to tighten one of the screws a lot more than the one on the other side. I was wondering why this is the case. Of course this means the spring on one side is working a lot more than the other one. This difference seems to get larger very slowly.

I even bought a used bike a few years ago and the brakes were so bad I actually had to use holes in different positions to install the V-brakes (one in the middle, one in the bottom). Worked well enough, though...

So is there anything I am doing wrong when tuning the brakes? Is there a problem if I change one of the screws by a longer one if necessary?

I usually leave one screw untightened and just tighten the other one as necessary, to minimize tension.
I suspect the brake arm with the tighter screw needs a good cleaning of its internals. Try spraying some penetrating fluid like wd-40 in between the brake arm and the pivot and see if that helps at all. If cleaning (thoroughly, when taken apart) doesn't solve the issue I can think of a few alternative reasons your brake arm might not be properly moving:


The cable and housing can pull the brake arms to one side (adjust cable routing)
(suggested by @Argenti Apparatus in the comments)
Mount bolt too tight on 1 side
Spring incorrectly installed (unlikely)
one of the o-rings seen in the diagram below is incorrectly installed/seated


Are you sure your rim is properly centered in between the two brake arms? If not your wheel might need dishing or it might need to be centered on it's axle.