This is a great question and I once thought about this for a while. Let me offer a few points.
First, these are two adages or proverbs. Therefore, we cannot take them as absolute truth. They are just sayings that people have made up in the past that reflect life to some degree.
Second, all people are different. So, for some people, absence makes the heart grow fonder; for others out of sight is out of mind. This is probably the most important point to remember.
Finally, if we introduce the idea of true love, things may get even clearer. In other words, if a person is truly in love, then the first proverb would probably apply. If the person is not really in love, the second proverb would probably apply.