Just the other day I had to clean out my bathroom sink drain as there was a bad clog.

After removing the "P" trap and cleaning out the piping there is usually lots of black slimy crud ( technical term ) built up inside the piping .... this is typical.

Here's one thing that I've noticed .... if shaving cream is being used while shaving this seems to contribute to build up inside the drain pipes. Usually this will be noticed on a pop up sink stopper as well.
That might explain why one sink is more so than the other.