Unfortunately they did not work out that one chip, like a unicycle is very difficult to balance on. ESP. if it is at a corner
so to fix this mess they did the only unreasonable thing.... added 2 chips.
Rather than this it's simple math, they do not have enough(any?) chips that could do 5(4.5 old spec) gh/s reliably so they were forced to use 2 chips to reach the performance goal. However most(or at least a majority) of their chips can probably handle 3,5 GH/s so there was no point in not offering this upgrade path.
Before someone goes bitching about that they should just have given out the upgrade for free, that would have fucked over the MR/S/LS customers since they cant offer the same kind of upgrade on those SKUs without using more chips.
They missed that by a mile. So 2x 2.5's made the difference.