Advance Purchase tickets aren't regulated or priced according to mileage or any set formula. They're set according to commercial demands.
If the TOC (Train Operating Company) who prices the ticket thinks that passengers are prepared to pay more, then the ticket price will be more. There is likely to be more demand from Birmingham than from Birmingham Int'l.
Another possibility is that the lowest priced quotas from Birmingham have sold out, but the lowest priced quotas from Birmingham Intl haven't, so you're comparing the cheapest fare from BHI with a fare from BHM which isn't the cheapest.
Because Advance tickets forbid break of journey, the Birmingham Int'l ticket cannot be used to start short from Birmingham, so the TOCs can get away with this policy.