That's the problem, most of developers think they are the same thing but they are not, an easy way to put is harem is a selfish version of poly.
Harem = The mc comes first, everything else is optional and if the mc want that (lesbian for example), the story is focused on the mc and him bonking.
Poly = Everyone fuck everyone of the group, nobody cares if one of your LI fuck another, they don't ask, they can be fucking in front of you, behind your back or you get in a room and they are playing and you join.
That is an example of poly. Harem don't have this, yeah, they can have fun, but the sexual part is just the mc, they don't think "he is having fun, let't me have fun too" is just "he is having fun, i'm happy for him".
Harem don't have jealosity, if they are in the group they love or at least care about each other, you don't put someone there that keep talking shit, fighting, you care about the group as much the group care about itself, the idea of the harem is "they are happy if the mc is happy", poly is "well, he is fucking, so let's do too".