Did the SE site originally allow a $250 minimum pledge? (I don't remember...) As of now, the SE site has a minimum pledge of $600.
If they originally allowed a $250, then I can only think that they were unsure of how much interest they were going to get, so they thought to offer a low option to allow more people to indicate interest. Also, we don't really know how many people pledged that low of an amount., so we don't know how many people that change will affect.
I will agree with you, though, that they should have synced up their SE minimum pledge with their planned offering minimum buy-in. I find it hard to believe that the offering wasn't already planned out prior to the SE page going live. Although there is always the possibility that the SE offering limit was an unintentional mistake. I'm not prepared to call it a deliberate bait-and-switch without some proof.