Let's try real simple.
Let's say you move a ZIP file across your dial up connection.
If you had software to unpack that and push the unpacked content across the virtual serial link, you could see a higher bit rate than what the actual link supports.
Hopefully this is enough of an explainer why you see bursts of speed. It's almost a bad decision to report these speeds to the end user since it most often ends badly with some feeling hurt over having to learn so much.