Different areas have different ways that it tends to work. Where I live they usually charge security plus 1 full month at move in and then on the first of the next month you pay the prorated amount instead of a full month if you move in mid month. This isn't how every landlord does it around here but most of the big ones do. It's because it's the way they can collect the largest amount legally possible before move in. Where I live they also limit the size of security deposits and won't let landlords collect more than 1 month's rent at move in. So instead of first and last and security, it's just first and security, so they try not to prorate first.

Everywhere has different rules, and different norms and traditions.