Because 6 colors x 4 bricks wide = 24 bricks wide for the rainbow and 5 colors x 5 bricks wide = 25 bricks for the transbow.

If only they had bought a little more paint, they could have gone 30 bricks wide and achieved perfection!

I think it's great the way they used the pixelated effect to lessen your trauma over the necessary change in width, although apparently not enough.

One has six stripes and the other has five stripes, so they have to use a different number of bricks for width if each stripe will be the same width. If you used enough bricks you could get them to be the same overall width, but then it would be too large for the space.