it is just because of lower volume / less traders.
If you see that the price at exmo is higher, just sell your bitcoins at exmo and buy them elsewhere

But of course sometimes there are real reasons for price differences. The most common reason for price differences are the fees, especially for deposit/withdrawal. So if Exmo would have high withdrawal fees for fiat, let's say 2%, the price would be ~2% higher at exmo, because people are buying bitcoins to withdraw them cheaper, than they could withdraw fiat.
But exmos fee is very low, so I think in this case it is just the small volume.

edit:
at the moment the USD price at exmo is really low, so you can make a profit by buying at exmo and selling elsewhere ..(so maybe it's not that easy to deposit USD at exmo?)   The EUR price is the same like elsewhere at the moment.