Some of us have habits or issues where we aren't so precise with our seam allowances, whether from cutting or sewing or whatever, lots of ways to get a bit off, especially when you consider there are two sides to every seam so doubles (or halves) our mistakes... With a log cabin you get lots of seams so lots of chances to go off even if all very straight.
I spent my first 20 or so years of quilting trying to be very precise in every step. The last 20 years I've been doing techniques where I "sew large and cut down" so I am sizing up my subunits basically every step.
You may have to trim some of your squares a bit to fit, one of the advantages to block sized square rulers. Also a good time to make sure the blocks are straight. It is ok if you have to stretch or ease in a bit here and there, but if things are measured and trimmed to some sort of consistent size, at least you know you aren't trying to stretch a slightly small block to an over sized block.