Even with the same card issuer (for example, I have two Discover credit cards of the same "model" for the same account), the chip always has a different outline. What's the reason for this?
Basically, the spec doesn't care. When you insert your card into a reader, there are six pins that will make contact, three along each edge. The area in the middle isn't really used, but should be connected to the upper-right pad (ground). As long as those six pins make contact, there's plenty of room for creativity. If your phone gives you easy access to the SIM card, you can pop it out, and see what the contact pins look like. (Fun fact: SIM cards and chip-and-pin chips use the same technology.)
https://en.wikipedia.org/wiki/Smart_card has a pinout, under "A smart-card pinout" on the right. Note that there are both 8-pin and 6-pin versions. The 8-pin version has 2 extra connectors for USB interfacing, which isn't usually needed, and so can be skipped.