This account already had 81 devices.
Apple’s Developer Center is now allowing certain developers to add up to 200 iOS devices to their account, double the previous amount allowed. Devices registered in this list can beta test a developer’s applications as well as the unreleased versions of iOS (and related software like iTunes).
Developers have been wanting the limit to increase for quite some time, since removing a device does not increase the count again until membership is renewed:
How many iOS devices can I register for testing and Ad Hoc distribution?
You are allowed to register up to 100 iOS devices for testing and Ad Hoc distribution per membership year.Please Note: Although you may remove a device from your account, it will continue to count against your 100 device limit.
As seen above, Apple’s documentation still reflects the 100 device limit and there has not been a new post in the developer center news, so the new limit may be a fluke or something that is rolling out to specific developers only.
(Thanks, Mr. Cool Guy!)