Probably while using Wireshark you saw MSS = 536 right? And did you ask yourself why this value?
Well, it is defined on RFC879 (https://www.rfc-editor.org/rfc/rfc879).
It defines that when not defined, the MTU should be 576, was consequence of this formula:
MSS = MTU – 40
We have MSS = 576 – 40 => MSS = 536