I am new to triathlons and searching for my first TRI bike. I found a 2018 Kestrel 5000sl SRAM Etap (Size 55cm) that a guy is selling for a very very nice price and I am really considering buying it. But, I was wondering about sizing and had a question I'm sure you guys could help with.

Why is sizing so different per brand? I am 5'11-6'0 tall with a 32 inch inseam. On most bikes I would be a 56cm but when I was looking on Kestrels website for sizing on the 5000SL it says for someone 5'11-6'2 I am a 59.6cm. A 55cm in their bikes is for someone who is 5'7-5'9.

Thanks for the help.