I agree they are pretty much the same thing - but their actual use may vary. I have seen the "call" and the "invitation" used in exactly the opposite way to what you have seen (i.e. call is general, invitation is to selected responders to the call).Hi
I understand the two sentences to mean roughly the same thing. Invitation to tender seems like a tentative attempt to attract bidders, whilst call for tender seems more insistent and might refer to a letter sent directly to potential bidders.