At my tennis club in the suburbs of DC, about half the players (when serving) call 30-up when the score is 30-30, and the rest call out the more intuitive 30-all.
To my mind, 30-up logically means the server has 30 and the receiver has zero. Note that the server's score is always given first. (Except in court tennis, see below.)
Wikipedia, Tennis Scoring System says:
.... when each side has won one, or two, points, the score is described as "15-all" and "30-all" (or "15-up" and "30-up") .....
So, according to Wikipedia at least, 30-up is an alternative term to the more traditional 30-all.
I played court tennis, aka real tennis, the predecessor game to today's tennis, for five years, and I never heard 30-up or indeed 30-all. I always heard the score fully called out, e.g. 30-30. Thus, 30-up (or 15-up) probably came into being in the past 150 years, after modern tennis was invented. (Real tennis originated in medieval times.)
I know language changes, not always logically, but this seems a particularly non-intuitive change, and I have wondered for some time how it happened and why 30-up (15-up) caught on.