is normal that it gives less than the current gemstone level
#Gemstones level
errant field
weary void
The range of attribute values of a higher level can overlap with those from a lower level.
(Similar with equipment.)
EDIT: Wah! My answer addressed the wrong aspect. Anyway, with Lem's correct answer, it is still "similar with equipment". (Difference is that there are 2 distinct sources: drops and crafting, so the overlapping tends to be worse than with gemstone levels.)
keen arrow
Just to confirm, there is a range of levels that you could get, so it is very possible to get a lower level gemstone off a pull. There is a filter in the Auto-Sell button that can get rid of these gemstones automatically if you don't want to see them.