Sometimes I notice that the B737NG starts to deviate from the FMC target speed during cruise, and when the deviation becomes too big it adds thrust to correct for this.
Happens to any airplane if you're trying to maintain a constant speed vs. a constant power setting(SEL aircraft). Hand flying, autothrottles, it doesn't matter. Speed variance is eventually adjusted when the human says "enough already". With a/t's it's the human that wrote the software code that decides when it changes the power.
Yes, the 737 wanders more than a lot of other a/c.