The third half is the bluetooth firmware on the other device. The fourth half is the other firmwares on the other device. The fifth half is the specification(s). The sixth half is the RF environment.
Haha -- yes, indeed. Bluetooth the spec and implementation are an absolute dumpster fire.
"Bluetooth is a layer cake of sadness" is the turn of phrase we used for a while on the Glass connectivity team. One of our project founders, Thad Starner actually apologized to me for the mess it became; apparently it was supposed to be simpler, but when Nokia took ownership back in the 90s, it started to go downhill.
Our lead on the connectivity team at the time had a crazy idea to rewrite the spec using only ACL and L2CAP, but never really went anywhere with it because of the the Glass org implosion.
A huge factor is that iPhone has a large, long-standing market share, with relatively few different OS and hardware versions. This means that everyone else has been able to test their Bluetooth implementations for interoperability with iPhones.
