prez, as I'm sure you understand there is no "ratings bump". What you are describing is a ratings report, which happens at the end of ST for every player (under the "Opening Day" line, most of the time).
The ratings report for ST shows ratings "improvement" from the beginning of the off-season to the end of ST, under the "Opening Day" line. When you promote a player during ST, the report generates a "Promotion" line, showing the player's "improvement" up to that event. The younger (and better) the player, the bigger (and better) the "improvement" on the report.
I know it helps me to think of ratings as "10.X" with "10" being visible on the player card and "X" being the incremental, hidden number. Assume that at the start of ST, all players have an even, non-decimal number in each ratings category. Depending on playing time, coaching, makeup, etc. that decimal increases until the next scheduled "reporting day" when the player card gets a new line. At that time, the new rating is reported - incremental growth. A player who is promoted (or traded) BEFORE the end of ST does not get an "Opening Day" report - the system reports the ratings numbers at the time of the event. So, when you say guys like Romano and Rossy improved despite no playing time, I respond "of course" because they were working out, working with coaches, etc. and were improving (according to their makeup, etc.) anyway. Increased playing time MAXIMIZES their development. But you can leave a top prospect (or DITR from the previous season) on the inactive list, at LoA, and he'll also show SOME improvment, because he wasn't sitting a the local Chili's sipping a beer - he was "working out", working with coaches, etc.
Put simply, it is NOT the event (promotion) that generates improvement. The event provides a (premature) report of the ON-GOING improvement of each player.