Fan speeding up over time

Has anyone else ever had a fan do this?
When I first got the fan MBM said it's average speed was about 5800rpm.
Now, a few months later it's saying it's up to 6400rpm now.
It slowly climbed, not all at once.
The fan is a Sunon 80mm 50cfm.
Doesn't really concern me, just wondering if anyone else has experienced this.

It might possibly be the calibration of the MBM is off a tad. I have the same fan and it is normally +/- 100 rpm.

Cant say it has happened to me. How loud is that fan now 6000+ rpm is alot...

IMHO the speeding up has to do with the deteriorating lubricant in the fan. It just spins more easily then before. Since the power rating of the fan hasn't changed, and the resistance of the fan has decreased, the motor is capable of greater speeds.