Like others have said, the route makes a big difference on average speed: hills, wind conditions, even the number of stop signs / lights have an impact.
I track my rides on a spreadsheet (retired engineer, I can’t help it), tracking time, miles, average speed, notes, etc. Average speed ride-to-ride doesn’t mean much but I have found that cumulative average speed seems to correlate with overall level of condition. It averages through the different routes, conditions, good/bad days etc., and as miles accumulate it’s harder to “move the needle” on cumulative average speed.