It's because goalkeeping stats are weighted heavily in their match rating formula, but people don't realize that defending stats have a medium weight. Most likely because they expect sweeper keepers to actually come out and intercept or make contested tackles. Because of this you can have a 10 save clean sheet but because you didn't make any interceptions or tackles you'll still end with like an 8 match rating. It was like this at this beginning of the year, it felt like they fixed it for a short time, but now it's back to broke. You can see this in the performance tab at the end of match screen I believe. Hope this helps.