PS/2 key reports are initiated by the keyboard, whereas USB HID reports are polled. Since (as the article describes) they wanted a fixed-size report for the ‘bo

PS/2 key reports are initiated by the keyboard, whereas USB HID reports are polled. Since (as the article describes) they wanted a fixed-size report for the ‘boot protocol’, the best they could do is report the current state, rather than a log of events since the previous poll. (Well, I guess you could drop the oldest paired non-modifier events, but that would be more complicated. HID just does a bunch of stuff that seems weird now to minimize device-side resources — e.g. HID descriptors aren't a structure defining a report, they're a bytecode for building a structure defining a report on the host.)