Top briefly updated too fast

While glancing at a top display from a Pi2 running buildworld
on -current the display update rate went briefly from its 
normal two second interval to several, perhaps five, updates
per second. In less than a minute it then reverted to its
customary 2 second interval. Top had been started with 8
as the number of processes to report. 

The top session had been running for hours (on the serial console,
if that could matter). No sign that anything special had happened.

I've never seen such behavior and wonder how it's possible....

Thanks for reading!

bob prohaska