After reading this thread, that shouldn’t surprise you.
I mentioned this back on page 2. The Wera Hex Plus keys achieve a greater contact area despite the looser fit. With this design, fit barely matters within a significant range, because the face of the tool is convex where it contacts the fastener. Whether it contacts on one part of the convex shape or a slightly different part barely matters: the convex shape (and the flats of the fastener) deform elastically and create a large contact area regardless, so the only detriment to a looser fit is the slightly lower lever length of acting on the flats slightly farther from the corners.
On the other hand, a looser fit makes fitting the key easier (quicker).
The probable reason Wera doesn’t include them is that they consider some inch sizes to be first choice and others second choice, as with metric sizes. First-choice sizes (e.g. 5, 6, 8, 10 mm in metric) differ from one to the next by enough that no undersized key catches the fastener flats. So, for example, a 6 mm key will spin in an 8 mm bolt head. Second-choice sizes (e.g. 5.5, 7, 9 mm) may engage, badly, in first-choice fastener sizes, tempting the mechanic to use the wrong tool with disastrous consequences. This risk is eliminated by not providing second-choice sizes in first-choice sets. That is probably Wera’s thinking. Of course it’s small consolation if you need the second-choice sizes.
