No it is not just psychology, it is the way search queries are written. If the very top of my budget is $15, I won't choose the" $15 and up" selection, I will choose the "under $15" selection. The $14,99 price will be shown and the $15 won't. Same thing if I write "<$15" instead of "<=$15".
Buyer's minds are busy thinking about product benefits, not whether to include the equal to case. Since they don't pay attention, you need to think about things like this for them. So even though they might prefer reading $15 to $14,99, if the search engine doesn't show the simpler alternative their expressed preference docent matter - their purchase behavior does.