A RAID Group does not have a RAID protection level until the first LUN is bound, so selecting the RAID level in the LUN Bind dialog is the most important.  When you specify a RAID Type for a new RAID Group, it is simply for reporting and filtering purposes at first.  I believe the LUN dialog will filter available RAID Groups to the ones that match the RAID type you specify in the LUN, making it easier to pick the RG you want.  Ultimately it's a UI decision made by someone to make something easier for somebody.  Just remember, the RAID Level in the LUN Bind operation is what matters.
I'm fairly new to SAN storage so please forgive the question.  I was just reading the EMC thin provisioning white paper and noticed that when creating a pool (or RAID group) you must specify a RAID level.  This is pretty obvious and straightforward.  Later, when creating a LUN which will be in the pool (or RAID group), it again asks for a RAID level.  Why is this?  Doesn't the LUN just use the RAID level of the pool or group it belongs to?   Thanks  Matt 
Pool/RAID Group RAID Level vs. LUN RAID Level