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It can vary widely based on what kind of window you have now (aluminum, wood, vinyl, etcl) as well as if you have brickmould casing on the exterior etc. It can be up to 5" different. Just let the professionals who are going to order your windows take exact measurements for you so they can take all of that into consideration. I agree with Michael that you should defnitely NOT order based on those measurements. You don't want to be repsonsible for purchasing wrong sized product if it doesn't fit. They can't be returned.
Did this just happen three years after installation or could this have been there from the time of installation and you didn't notice. Sometimes this can happen, especially on older homes that have old wooden windows where the installers use something like a chisel to literally "bust out" the old exterior blind stops on the window when installing replacement windows from the exterior. The old wood can be very dense and hard to chip off. The force of hitting on the exterior could cause this type of thing or as someone else said, could have a shim putting pressure against the back side of the interior trim. You probably would have noticed that kind of damage right away though.
Typically a 70 year old house has already settled so you wouldn't think that was the cause, unless you are having some other kind of structural problems with the home. If it were water, I would imagine you would see some kind of staining or drips on the interior as well.
It's really difficult to say, unfortunately. It could be installation related, but would be hard to say without pulling off the metal wrap on the exterior (assuming they were wrapped) and seeing what is going on.
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