ID’ing verbally as gay is easier in a lot of circumstances as it's generally more understood. In some spheres, gay is simply used as an umbrella term for all Queer, LGBT and other gender and sexual minorities. So according to this understanding of the word, being pansexual is simply a “sub-category” of “gay”, and a pansexual can easily be both. Pansexual does not have the same well-established so people use “gay” as an identity-aiding label on their sexual preference.
* Pansexual - people who are attracted to both non-binary genders and binary genders based on emotional, romantic and sexual feelings.
** Not an expert, still learning. I still make mistakes, hope I haven't offended anyone.