Girls, why would you show interest in person but be cold while texting?

This girl flirts a lot in person, but when we text each other (it's our main form of communication since we live 2 hours apart) she sometimes won't respond and won't ever text first. Our conversations are usually short, but she seems pretty interested in them.

Any idea what's going on in her head? It doesn't make sense to me at all.
Girls, why would you show interest in person but be cold while texting?
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