Why do some girls take so long to respond even if you message them back right away?

I've been talking to and seeing a few women. Some respond within minutes or seconds and some take hours to days to respond. Why is that? Like I was talking to one girl she told me she'll let me know in a few hours if I can come by. Doesn't say anything then messages me last minute to come and I said no it's to late then she
Gets upset
Why do some girls take so long to respond even if you message them back right away?
Post Opinion