30% is one of those mathematical “sweet spots” that is created by the human perception of numbers. This perception puts a ’snapping effect’ on prices, so that 40% sounds like 50%, which sounds like a hefty raise. “50% increase!” never sounds like a lowball, and 40% gets chucked in the same bucket because it’s easier to think about it that way. So the perceived difference between 30% and 40% is a lot more than the perceived difference between 40% and 50%. Same thing happens with 10% (sounds like nothing, pocket change, sales tax, etc.)

20% and 30% tend to group together and people tend to perceive those as nearly the same as well, and think of them both as in the ‘modest’ range. So that’s the sweet spot, and the manufacturer is going to choose the higher of the two numbers since it’s a perceptual freebie.

As a result, 30% over-and-above price (above $10, above $1, above 100%, whatever) are disproportionately represented and I think this is a pseudo-coincidence caused by two retailers each playing out similar logic about user perception.