Unofficial Skyrim Legendary Edition Patch

I wouldn't bet on those numbers meaning what you think they mean. Have you actually tested those values to see if they produce the expected results? Bethesda's formulae are frequently counter-intuitive.

If you're referring to what BowSpeedBonus itself means, you are right to be suspicious. The actor value's name doesn't tell you much and is kind of misleading. I tested it in-game. What it really means is "time multiplier while zooming in with a bow". BowSpeedBonus = 1 means time passes at normal rate, and BowSpeedBonus = 0 means time is stopped while zooming (although this can cause freezes). Also, unlike the Slow Time shout, the time multiplier is the same for the player as everything else.
If you're referring to the perk ability magnitudes and how they work, I've confirmed that BowSpeedBonus is modified as I described by querying it in-game ("player.getav BowSpeedBonus" before and after getting each rank in the perk.

Looks like you've done the research then. Given how the values are set, and how they stack, I think the magnitudes are correct, and it's the descriptions that should be changed. Whoever wrote the descriptions probably saw the .25 and .50, and didn't look too hard at which was which, but whoever set the magnitudes appears to have understood that they would stack.

Fixed for USKP 2.1.2.

Text changed to match the actual effects in game.