I've just created a view that summs up percent value, that is saved in my SQL back-end as Decimal (5,4).
It resulted in the datatype (DECIMAL (38, 4)). Those 37 non-necessary digits were read by the Access database as Text, not number. If the number is treated as Text, then that causes ceveral problems to the access database: for example, it cannot format it as percent.
Since I don't really need those 37 digits, I've changed it to Decimal (6,4) (with an extra digit just to be on the safe side ; the sum shouldn't anyway exceed the 100%) and re linked the table. That sort this problem out.