Part of the problem could be that gas expands when heated like most other liguids. Gasoline is stored in underground tanks making it much cooler than ambient temperatures in the summer. If you live in a particularly hot state and you have filled your tank to the absolute max you will get some overflow as described. The opposite is true in winter (especially noticeable here in Canada) where the gas is much warmer underground and freezing temps can lower your gas level in the tank as the gasoline contracts. We're not talking about big changes in volume, but for a freshly filled 20 gallon tank it is not surprising to me that you have had this problem. The smell of gas is the first sign of danger however, and unless you are 100% sure you have found the source of the leak and this is normal I would have a qualified tech checkit out.
1995 E420 - retired
1986 420SEL - retired