i just read something about this actually. According to google, an impression only counts if the visitor presses enter or clicks off of the search bar or sees the results for no less than 3 full seconds. To google that counts as a search.
So "windo" could be a number of things. it could be that people typed that in and then pressed the search button or enter. Or it could mean people started typing "window" and clicked off after "windo". Or it could mean that they startted typeing "window" and before finishing they started to look at the results and stayed on the SERP for longer than 3 seconds.
hope that helps.