Our way to derive a nonce from chunk counter and IV prevents a crucial OCB optimization
https://openpgp-wg.gitlab.io/rfc4880bis/#section-5.16-15 says:
The nonce for AEAD mode is computed by exclusive-oring the right-most bits of the initialization vector with the chunk index as big-endian value.
Note how incrementing the counter does not result in the nonce being incremented, i.e. the nonce is not a counter. However, a crucial optimization in OCB relies on nonces incrementing, see https://www.cs.ucdavis.edu/~rogaway/ocb/ocb-faq.htm:
OCB uses an amortized m + a + 1.016 blockcipher calls, where m is the blocklength of the plaintext M and a is the blocklength of the associated data A. This assumes a counter nonce. (In the worst case, when the nonce is not a counter, the 1.016 constant becomes the number 2.)