VPI SDS owners-

I check the speed with the appropriate strobe every 4-6 weeks, or when I make a cartridge change. It was initially set at 60.12 when I got it (about a year ago). I would say I had to lower the speed 5 or 6 times since I got it. It now runs at 59.53 HZ to be spot on. Anyone know why this is happening? Should I be concerned? Turntable is the original VPI Aries1.