Because the bolt bodies can vary a good bit in length. This is what makes the Savage so cheap/easy to build - they don't have to precision fit each piece or bolt assembly to each specific action. If the bolt body or bolt head is a little long or short it doesn't matter as they can account for it by simply screwing the barrel in or out a little before tightening the nut.

Savage has several different outside vendors supplying each part of their bolt assembly and there are built in tolerances, so expecting a replacement to be the exact same length as the original is wishful thinking. There have even been a couple instances where bad batches of bolt bodies that were grossly to long or short made it out to the public as "replacement parts".ALWAYS check your headspace when replacing any part of the bolt that could affect it's length: Front baffle, bolt body, bolt handle and bolt head.