Why, indeed?
The statement isn’t entirely accurate in any case since all special resolutions are conducted as poll votes and so it would be 75 and 25 percent of unit entitlements held by those voting …
I suspect it’s worded that way, partly to help eliminate the abstention argument: If you are abstaining then you are not voting, and therefor your vote is not counted when calculating the percentages.
Even so, it might just be a way of emphasising the status quo, which remains if more than 25 percent vote “no”.