Why do some girls use "<3" instead of ❤?

Why do some keep switching back-and-forth between those 2 heart emojis? I get that they're supposed to mean the same thing, but is there any specific reason for why a girl would rather use "<3" in a specific situation rather than ❤ (assuming that the girl uses both at different occasions)?
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Oh oops, I meant like when girls type it as "less than 3" (< 3). Can't write it properly or else it will turn into an heart emoji.
Why do some girls use "<3" instead of ❤?
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