There is more than one reason why horn scales can be bent. They may simply have simply taken a bend, which is about bound to be the case if they are both bent outwards, or both inwards. Another possibility is that one scale has simply expanded or contracted more than the other. This is very likely to be the case if one scale remains straight. Either condition could be responsible if the concavity is on the same side of the razor. One reason for uneven expansion or contraction could be the razor lying for years in a place where one is damper or warmer than the other. I don't believe there is any practical method of making one scale contract lengthwise.

It is a good idea, if you are dismantling a razor, to remove one rivet first, and see what happens. You may find that when you do this, and straighten the bend, the empty holes are slightly out of line. Surprising little is enough to produce a bow in the scales, and you may find that widening the holes into line and using a larger diameter rivet is enough to cope. If the materials allow you to reassemble the rear end of the handle with epoxy, you may find that a full-diameter 1/8in. pin, with its ends disguised as the washer and head on a 1/16 rivet, is all the fastening you need.