Isn’t that just because on the ladder the time is measured in real time seconds, but in coop it’s Blizzard seconds (=72% of real time second on Brutal) and the speed of a unit might be measured as distance over said second. Void ray had speed of 2.25 in WoL and HotS ladder when time was measured in Blizzard seconds
edit: the LotV speed equivalent of 2.25 is 3.15 and it has been increased slightly since then: