Although YF and YO are NOT the same thing, some pattern writers still think they are interchangable. In addition, in some countries, yf IS a yarn over.
I'd bet money that this should be a YO, seeing as you're coming up short on stitches. A yf before a knit will create a yarn over anyway. A yf before a purl will NOT create a yarn over. There are 3 of those in this pattern repeat, hence the 3 stitch shortage for the next row.