Pete N4ZR has written a blog entry on the Reverse Beacon Network blog that gives advice to those operating spotters and those wishing to be spotted.
"Make sure the CW is good – letter formation and spacing, and 3:1 dot/dash ratio. Any errors here may manifest themselves as copying errors. Skimsrv is pretty forgiving, but there are limits. Above all, avoid using half-spaces right before or right after your callsign."
Seems to me that among the most common sending errors are failures to provide adequate spacings during calling sequences like CQs.
My RBN spots are invariably correct and the speed observed corresponds with mine. I use a bug whose speed I estimate by matching by ear with a desktop keyer. A wpm off a couple of characters
is unimportant. In the same class as "S9 plus double digit dB or an "SWR" designation two digits to the right of the decimal.
I think the speed observations have only limited utility simply as a means of helping to ID an observation which otherwise might contain errors.