The only thing it proves is Rafa was the virtual #1, though someone else was the statistic #1 [which is accrued with even idle weeks and smaller wins through the year].Not really. Nadal was in his prime and #2-3 in the world. Same can't be said for Federer and Djokovic.
This stat only shows how Nadal failed to be #1 for a longer perioid of time.
Off the top of my head I'd say most of those "wins" for Rafa were at RG, where he has been seeded #2 or #3 many years because of the odd ranking on clay. It was common for him to arrive at RG not the front runner on paper when obviously he was dominating on clay. Just check how often he did not have #1 seeding at RG in years he won.
And no of slamsSlam records while ranked #1
129-11, .921 overall
11-5. .688 in finals(3 losses to Nadal at RG)
160-14, .919 overall
13-7, .650 in finals(2 losses to Nadal at RG)
94-10, .904 overall
7-2, .779 in finals
Slam matches played while ranked #1:
Slam finals played while ranked #1