Saddle Comfort

 I am so much in love with my Brooks B 17 that recently I swapped the Surly's saddle with the Red Roadie's. Since I was not riding the LHT and was taking out the RR every evening I wanted to go for the more comfortable seat on that bike. The LHT had the Brooks while RR had the Shimano. To keep the swap process simple I took out the entire seat post and placed one into the other, so that swapping it back would be easy, if I need to. 

While the Roadie became super comfortable, despite its geometry being different, the Surly has become super uncomfortable (I use it for the short office commute once in a while). Now, I am a little puzzled by this. I have ridden the Surly with the Shimano seat on before. In fact before the arrival of the Brooks that was my installed seat on the Surly. It wasn't so uncomfortable then. I went to Darjeeling in this combination. So what could be the reason for this new found discomfort?

I have two possible explanations. A. I have got used to the comfort of Brooks B. The seat post has a major role to play in saddle comfort. 

The LHT seatpost is a stainless steel one, while the Merida seatpost is made of aluminum alloy. But I must also add here that the Red Roadie with the Shimano seat and Merida saddle wasn't as uncomfortable as what the Surly feels now.