For anyone wondering, his open play league goal numbers by season are actually 19, 17, 19, 3, and 19, the 3 obviously coming in the 2004/05 season that was ruined by injury. That does indeed produce an average of 15-16, if you not only ignore the context of the injury but also commit the error of weighting all five seasons equally when one of them accounted for roughly half as many minutes/appearances as the other four.
I would instead phrase my interpretation of these numbers as "in five years at United, Van Nistelrooy averaged a non-penalty league goal every 158 minutes, which would work out to 21.6 per season - elite goalscoring numbers that are comparable with most top strikers of the Premier League era and likely the best among United players"*.
I had to double-check to confirm this was the same poster who said earlier in the thread that Van Nistelrooy scored 4 CL knockout goals without even bothering to include the number of games. Not sure what's going on here!
*edit: a quick check reveals that he's ahead of Rooney and Van Persie but narrowly behind Ole (153 minutes per non-penalty-goal - what a legend). Cole (who deserves massive respect for amassing 187 Prem goals without a single penalty) is behind Ruud across his entire career but is most likely ahead if you exclude his post-United spells where he was past his best - can't be arsed to pull the numbers. As anyone would know without needing to read my post or the spreadsheet above, Van Nistelrooy was an elite goalscorer even without the penalty stat-padding.