All fighters are subject to random drug tests at all times. I just checked USADAs testing history and Anderson has been tested twice in the last three months. Side note: He's been tested a shocking 11 times this year. That is a lot of testing!
Anyways, I think that you misunderstand the USADA program.
Every fighter is testable at any time while under UFC contract. They are required to inform USADA of their whereabouts frequently.
Fighters are also tested on fight night per normal athletic commission procedures.
Fighters are supposed to be in the USADA testing pool for 4 months before any fight with the UFC. I think that this is what you were alluding to. This just means that they are part of the program not that there is any particular schedule of testing that has to be followed.
Lastly, that four month rule can be, and frequently has been, waived for fighters who make short notice debuts in the UFC or short notice returns. Recent examples are Brock (who has been tested 6 times since it was announced that he was coming back) and the guy who fought Khabib on a weeks notice.