Probably because they're different projects, written by different people, in different languages (predominantly C/C++ versus python), with different setups and configuration.

Your question is like asking why Firefox does something when Internet Explorer doesn't - they're not the same program, they're written by different people, they're not doing things in the same way - and it's quite frequent that a website change can work perfectly in one browser but break horribly in another.

The new www2sms is working fine with the Exetel provider, so I'd say the issue is specific to the O2online.de provider. It's entirely possible that their website has changed in a way that affects one and not the other...that said, there's also been a Python QT update for the phone in the last week that may also have affected www2sms (and probably not affected the O2 provider implementation in web2sms since they don't use python for the majority of the program).

Honestly, if you prefer web2sms, use it. Having the 2 programs gives choice. For me, the Exetel provider in web2sms is broken, and the string of spagetti code to thread to fix it is a nightmare I really couldn't face - plus fixing some of the Exetel provider issues in web2sms means breaking other sms providers *wrinkles nose* which is not something I really wanted to do. Implementing the Exetel provider in www2sms was a better choice for me - and I got significantly more functionality for the provider by doing so than the web2sms implementation enabled.