What's missing in BlackBerry 4.6 browsers that gives them a C-grade?

What's missing in BlackBerry 4.6 browsers that gives them a C-grade?

Reviewing the device compatibility chart for jQuery Mobile, it looks like BB browsers 4.5 and 4.6 get a C-grade, but the 5.0 browser gets an A.  RIM's documentation shows that the 5.0 browser has "partial" support for JavaScript 1.6, but the earlier browsers don't.  

Does anybody know whether jQuery Mobile simply requires the JS 1.6 features in the 5.0 BB browser?  Or are there other features that the older browser don't have which give the C-rating?  Do any jQuery Mobile features work on pre-5.0 browsers, or is it just generally incompatible?  We might be able to work around specific unsupported features (maybe we wouldn't even need them) but if it's a complete non-starter on pre-5.0 we'll have a lot more thinking to do.

Thanks for any help anyone can offer!