Ah I see. Well, there doesn't have to currently be lots of 10th level alchemists, but they just have to have existed across the world since the beginning of time.
Well, most will be lost after some time - lost at sea, destroyed by violence or accident, buried with the honored dead, buried under a volcano eruption... even if you assume all magical items are immune to the forces of time (which even bronze swords are not, much less those made from iron or steel that isn't covered in crome or something like that), you will need a steady influx of new stuff to replace the lost ones.
And remember, the same forces that inhibit leveling up in NPC's now will have done that at all times, so the overall rarity of magical or even just masterfully crafted items will be comparatively higher than on worlds that regularily see higher levels.
Low level NPC populations and high level magical items... they just don't seem to fit each other in my view.
Also, consider the affect if you use Ritual Magics to allow multiple alchemists to collaborate.
That would imply that there are enough of them at high enough level to meet each other and having time to do the stuff together.
All the items are in crypts.
Actually I would posit that the higher level bonus items (15/20/++) are less likely to suffer from the elements and hence can be found intact (as usually happens in literature of all sorts).
The level spread of the population wouldn't necessarily be the same across the history of the world. So the current level spread in a specific place/time would not apply elsewhere (say for even for example long-lived races vs. short-lived races or peaceful vs. violent cultures or one plane(t) vs. another).
Given the business of alchemy, I could see an Arcane Guild (or a country and/or business) actively assembling Alchemical teams to build items. (The ROCO III Ritual rules work well with casters swapping out - on-shift/off-shift - plus Alchemical rituals can be suspended easily.) Actually, if I recall correctly, a 10th level Alchemist with 10-20 ranks in Alchemical Ritual, probably can easily do a 20th level spell via ritual. So +20 bonuses are not out of reach.
But, as usual, YMMV - you can always expand the level ranges and/or raise/lower percentages to your liking.