base64 encoded e-mails and compression

base64 encoded e-mails and compression

I sent an e-mail which included almost 2500 characters of text and one
attached 60KiB PNG file. This text included number 185 eight times.
However, after the e-mail was encoded to base64 and sent, I'm able to find
only three "MTg1"("MTg1" is "185" in base64 encoding) strings from e-mail
source. How to explain this? According to e-mail header, content type is
"text/plain; charset=UTF-8" and content transfer encoding is "base64".