Yeah things have changed with the addition of new speeds.
When there was just 1 speed, power was determined on a very linear scale.
A 10 ft. putt, no elevation, you'd hit it about 12 ft. (to be firm)
20 ft., no elevation = 22
20 ft., 2 in. downhill = 20.
20 ft., 2 in. uphill = 24.
100 ft., 2 ft. downhill, = about 75 or so.
Then "Fast" came, and people flipped for a few days, it was no longer linear. Any thing uphill or downhill became a lot trickier. Power was suddenly determined on more of a logarithmic scale. i.e.
10 ft., no elevation = 10
20 ft, no elevation = 18~19
30 ft, no elevation = 26~27
40 ft, no elevation = 34~35
50 ft, no elevation = 42~43
100 ft, no elevation = 77.7676ish
(these numbers are just relative guesses really, I go more by feel)
The longer the putt the faster it plays, especially downhill.
You all know this, of course, or have your way of figuring it out, I'm just using it as an illustration here.
Because "Standard" isn't a practice option (now that's gotta be a bug!) I can't really test this postulate, but the new "standard" seems to be slightly logarithmic in scale, as opposed to completely linear like it used to be. But unlike the faster speeds, you have to hit it a little harder for longer putts. So your 10 footer is still 12 ft. (to be firm) but your 50 footer is 55~56 feet - behaving more like a SLOW green. And SLOW is now even SLOWer
Just a theory though, and roughly matches tibbets post here.
Why it changed I'm not sure. Is it a BUG? debatable.
WGT certainly isn't going come out and say the 'Standard' is Log base x and 'Fast' is Log base y - cause some hacker would fashion some virtual abacus and try and sell it for $10.