The simple reason is that the Rem cases are thinner than Winchester so have a greater internal capacity, you do notice this with some cases, we worked up a good accurate load for my mates 6.5x55 swede, and with a load that was fine with, I think Lapua cases,(could have been Hornady) they were over pressure in Privi cases, so it pays to start low and work up, when changing any components, as for the case flare die, well I always trim/chamfer the case, and I tried both with and without case flaring, and noticed an improvement when flared, I think because Hornet bullets are flat based, the flare