*This post's info is invalidated.
~~So I just took a look at the Rengar and Rammus' items. ~~
~~Could've used a bit more info, like a link to the actual League Match History page so I can see build times, or an actual replay, but check this out: ~~
~~http://na.op.gg/summoner/userName=Gojira23 ~~
~~The Rammus only had Thornmail. How can you possibly claim he had 500 armor? This is spreading misinformation just to get Rengar-haters to view the thread. ~~
Here's the Rengar's build: http://na.op.gg/summoner/userName=Hansol
~~If these were the end-game builds that clashed with one another, it's no wonder Rengar didn't even need a LW to kill Rammus; Rammus was building mostly hp and resist for zoning... Veigar? ~~
~~Don't forget that Rengar could've burst Rammus down BEFORE he used Defensive Ball Curl. ~~
tl;dr OP posted exaggerated information. Rammus' build had Thornmail as his ONLY armor item, the rest were mr/hp.