The 'limited offer' rewards are designed to give early backers a better deal.
The difference is that while you also get every 'non-limited offer' below that they do not 'stack' like the basic offers.
So if you pledge the $45 T-Shirt, you get the $2, $10, $20 & $45 rewards as well but you don't get the $35 'limited offer' as well. (You don't get the $12 limited offer but in that case it doesn't matter because you get the $20 reward which has a lot more anyway).
if you pledge the $35 T-Shirt reward you get the $2, $10, $20 & $35 rewards.
If you are not sure then you can look at the pictures in the Rewards section to see drawings of exactly what you get for each pledge.