Birthsign abilities increase modified attribute values instead of base ones
Attributes in stock Morrowind are capped at 100. If you choose the Lady as your Birthsign, you start with +25 Endurance and Personality, but can still only max out at 100.
As per the attached screenshot, OpenMW let me increase my Endurance to 105. The last time I leveled, the dialogue let me add a 5 point bonus to an Endurance of 75, for a total of 80. Note that the stats menu shows that my Endurance is highlighted (magically enhanced) by the +25 of the Lady. Note also that I selected Strength and Endurance as my favourite attributes, but those buffs don't show as highlighted.