I called EGL USA today because it sounds like this will be a recurring question (and b/c they never answered my email!!) - EGL LA office told me the difference was ''simple'' and put me on hold. After a while I was someone else picked up, I repeated my question asking if she could explain what the difference is between ideal and ideal plus, and she hung up! (Hopefully the hang up was an accident! I am really curious and was not trying to be offensive) I then called EGL NY and after a while got through to a very nice lady who explained very politely that there is an industry standard and that it is scientific, and that if I was interested she would be happy to email me the specs on Monday as they are about to close. She also brought up to be sure to confirm an EGL USA report if looking at a particular stone - she said go to eglusa.com and go to online results, then type in the report # - if the report comes up then great, if it doesn''t, then they should be contacted b/c she noted there are a few fradulent reports out there.
I will post the results when I get them to clarify for everyone.