They are not that similar - soloist has longer chainstays, less front centre and much longer Head tube.

If you want an all round bike then the soloist is a good bet, it's not designed to be ridden as steep as the P3 is, nor is it as easy to get low in the front. If you are looking at getting one I would suggest having the store set you up in a road position and in a tri position and get them to show you how to make the swap.

I have noticed this as well, many brands have dissimilar sizing between models, my 56c P3C and 58c Trek SSL have similar top tube lengths