It "left out" the A in its decode and still correctly answered the proposition, either out of reflexive familiarity with the form or via metasyntactic reasoning over an implicit anaphor; I believe I recall this to be a formulation of one of the elementary axioms of set theory, though you will excuse me for omitting its name before coffee, which makes the pattern matching possibility seem somewhat more feasible. ('Seem' may work a little too hard there. But a minimally more novel challenge I think would be needed to really see more.)
There's lots of text in lots of languages about using an online base64 decoder, and nearly none at all about decoding the representation "in your head," which for humans would be a party trick akin to that one fellow who could see a city from a helicopter for 30 seconds and then perfectly reproduce it on paper from memory. It makes sense to me that a model trained on the Internet would "invent" the "metaphor" of an online decoder here, I think. What in its "experience" serves better as a description?
I remember running this experiment some time ago in a context where I was certain there was no possibility of tool use to encode/decode. Nowadays, it can be hard to certain whether there is any tool use or not, in some cases, such as Mistral, the response is quick enough to make it unlikely there's any tool use.