SDS question

Hello friends,
I was checking my turntable speed today using my KAB speed strobe and found that my table was running quite slow. I had the VPI SDS calibrated at 59.85hz and I thought I had the speed perfect. Today, I needed to adjust it to 60.03hz to get it dead accurate again.
What might cause such a big change to have to be made? I was wondering if it might be the weather (it has turned cold recently here in Chicago)?
Any thoughts would be appreciated.
Its been a while since I was up to date on TT technology. What kind of motor is used? A synchronous motor, common decades ago, would assure that the rpm was exact ("dead nuts" is the technical term). Bearing lubrication, temperature or phase of the moon would have no effect. Only a change in line frequency would affect speed.
Start with the lube and proceed to the belt. I think keeping the bearing lubed may be the most overlooked "tweak" or maintenance procedure. Throwing a few drops of factory-approved oil in the bearing made my near-new Technics SL12x0 spin much nicer and a little quieter.

Belt wear and slippage is a continuous process and usually doesn't get addressed until it's obviously audible. By then it makes the turntable sound like it's seriously broken. But one would think intuitively that a belt could wear and slip the equivalent of .18Hz over time, and one wouldn't notice unless strobing it.
Just following up on Starcons' comments, I was at 60.54 before hooking up my PS Audio PPP. Now the SDS is at 60.00 and my turntable sound is noticeably improved.
OK...I forgot belt slipage. I am a DD guy where the speed of the platter is the same as the motor, and the motor is exact. I really don't think that there should be any belt probably need a new belt.
Thanks for all the responses. I lubricated the bearing with white lithium grease about 6 months ago, so I should be good there.
The belt is the original belt so it is about 3 years old. Time for a change?
The motor is also original, and it does seem a tiny bit noisier than it was 3 years ago. How long should a motor be expected to last?
Actually, I was wondering if the how much an acrylic platter would expand/contract with temperature changes. Is this a possibility? The room containing my TT gets very cold when I'm not home and the heat is off.