This is odd. Apparently, there's a trust difference between the two UA. I first thought that Verisign root CA wasn't available in the store used by wget, but wget downloads https://www.verisign.com/ fine, and both sites have the same certificate root "VeriSign Class 3 Public Primary Certification Authority - G5" (serial 18:DA:D1:9E:26:7D:E8:BB:4A:21:58:CD:CC:6B:3B:4A). The difference may lie in that www.verisign.com is providing the three certificates, but mentor.ieee.org only the last two.
That for the certificate failure. The problem you get when using --no-check-certificate is due to https://mentor.ieee.org/802.18/dcn/12/ not containing a file list. TRy going to https://mentor.ieee.org/802.18/dcn/12/ in your browser: "Not Found The page requested was not found or is not a functioning properly" Obviously, wget --no-check-certificate https://mentor.ieee.org/802.15/dcn/12/15-12-0290-01-0009-tg9-closing-report-may-2012.ppt works properly.
