because the letters are, presumably, a bit shorter than the average across 46/47 characters. So I'm actually attempting 20/16.8 !
Yes, the effective speed is calculated from the number of characters sent and the time it takes to send them, and when a short letter is the new one (or the mix is weighted towards shorter letters) then the effective speed can come out a fair bit higher than the speed you've asked for. The reverse also applies, of course, once there are longer-than-average characters in the mix.
once you get enough letters going, the spell check starts 'fixing' things as you type them in, which won't help your scores :-)
Once, way back, a colleague inadvertently allowed a piece of software to "correct" the spelling in a technical document that had taken a month to write. Of course, the software didn't know most of the technical terms, so it just replaced them all with words it thought were close enough. The result was hilarious to read, but not technically much use...