There's a million reasons why they might have done this. Experience might have dictated that your 15kt headwind regularly turns into a tailwind during the rotation. The winds could have been forecast to change so they went for the more conservative option. Could just be an element of airmanship to round down on the wind (although I admit that this is perhaps a way too far, 15kt headwind might reasonably be rounded down to 10kts to allow for a bit of variation in the w/v).
I assume in this case, it was probably just some habit they've developed, or perhaps they were trying to show off about how their enlightened conservationism was the superior method, when in reality it was perhaps not really a well considered request.
Did you not ask them why they wanted such conservative figures? Perhaps they had a good answer?