Measuring TT Speed

In measuring my tt speed I am seeing the lines on the strobe bounce back and forth, not holding steady. This is a belt tt with a dc motor setup with a closed loop controller, and ideas on what can cause this? I am thinking the belt may be to tight, it is a new belt BTW...
The RPM Turntable Speed Accuracy/World TT Database app for iPhone has a way to calibrate the app and then applies a multiplier for better precision.

The RPM Speed and Wow app and the RPM Calculator app both for android, which are popular, dont have any way to calibrate the app, last time I checked.

Have you ever been aware of a speed change when listening? If not, try to ignore it.

Wonderful Thorens TD 124, idler wheel drive, mirror constantly reflecting strobe on underside of platter: each session the motor had to warm up before you adjusted it to get the built in strobe lines steady. then after a while, check, might be a speck slow or a speck fast.

Not that I heard it, just when changing lp's I saw it. And that's a 7lb or 9lb platter to behave like a flywheel, with the most precise bearing I have ever seen.

Direct Drive, not quartz, similar, warm up needed, but steady after that.

Direct Drive, Quartz Locked, ON, walk away.

The speed controls are nice, but wait 10 minutes for warm up, and then only a minor adjustment should be needed, their larger range is for purposeful audible changes.
I have yet to see actual W&F measurements for a DD table that is any better than most other tables. Published W&F measurements from the manufacturer, sure.
I replied to the OP already but I thought I'd add a comment about the calculations for platter speed from the KAB USA website. Please correct me if I have this wrong, but the platter speed calculation didn't mention that when the platter is running slow you have to account for it by either using a negative error, i.e., [1 + (-count/360)] x RPM, or, subtracting the error, i.e., [1 - (count/3600)] x RPM.