Should also note that double rounding cannot account for the _original_ symptom here.  Double rounding surprises on Intel chips require an exact product at least 65 bits wide, but the OP's sequence is far too short to create such a product.  (Steven's 2049 just barely requires 12 bits, which when multiplied by a 53-bit value can require 12+53 = 65 bits to hold the full product.)