A high Finishing attribute does not mean they score more goals. Other attributes collectively can contribute to goalscoring such as; Technique, Anticipation, Composure, Concentration and Off The Ball. On top of that you have other factors to take into account, physical / mental attributes, team, tactics, league etc
I'm guessing Weghorst's finishing is higher than Rashford based upon him historically scoring more goals recently in his career compared to Rashford. Since 2018, Weghorst has scored 71 in 161 games, Rashford has 56 goals in 151. Plus Weghorst has been 'scored' by 3 different researchers for 3 different clubs across 3 different leagues in the last 3 years, presuming the Man Utd researcher didn't change anything in January as the sample of games was too small. Therefore the last meaningful research was for Besiktas where he scored 1 every 2 games.