"I saw"= past tense
signed= past tense.
But for me to 'see' that you had done so, you must have done the actual 'signing me up' before I saw/found out about it, at some point earlier in the time frame.The 'had signed' construction places it further back in time.
I see (present tense) that you signed (past tense)me up then, even though I asked you not to.
I saw that you had signed me up, (so it saved me the trouble)
Part of the problem is that in speech, with the contraction, "I saw that you'd signed me up", the 'd of you'd gets so "swallowed", that sometimes we lapse into saying, I saw that you signed me up.