The PCMCIA PC Card standard only defines a specification for the pin configuration and the height of the card. The most common variations are the width of the card and rounding of the corners. If you have problems with the fit of a PC Card, try using the bottom slot.
The specification defines all dimensions for a Type II except radius of corners. In reality, not all cards (but most) conform to the spec. Some modem Type II specs exceed the 5mm height. The 5mm is a maximum.
What is undefined for Type III cards is the top case minimum dimensions. The maximum is defined, so cards that conform to the maximum work with the connector. Variants of the spec (unusual shapes and divots) in the top case can cause issues.