UN was good but he didn't know everything. Read the below link and explanation of EB2 category.
http://www.shusterman.com/employmt.html
It clearly says the following
EB1 = "Priority workers (28.6% of the worldwide level of visas, or approximately 40,000 visas PLUS unused special immigrant and investor visas, if any)"
EB2 = "Professionals with advanced degrees and persons of exceptional ability (28.6% of the worldwide level of visas, or approximately 40,000 visas PLUS unused visas from priority worker category, if any)"
Statistics and calculations don't mean anything until the facts are correctly followed. I would expect members on this forum to do their own research instead of taking for granted other member's speculations and blindly following what they hear and read.
Conclusion: Unused EB4,EB5 and EB1 numbers will flow into EB2. If you know differently show me the proof like I did. Don't give me bull about He said... She said...