That's a great bit of technical expectation management.
"We'll accept up to 25MB!" And then when you send something 26 or 27MB because you don't notice that it's so close to the limit, the system forgives you.
More likely it is an accommodation of base64[1] (MIME) encoding of binary files, which results in a 30% expansion (3 bytes get encoded to 4): 25MB * (4/3) = 33MB. People who read that GMail is limited to 25MB will expect their 25MB photo to be accepted. This requires the absolute size limit to be set to 33MB (probably bumped up to 35MB to accommodate the HTML body and other spurious stuff like the 25 off-topic replies).
"We'll accept up to 25MB!" And then when you send something 26 or 27MB because you don't notice that it's so close to the limit, the system forgives you.