...between Darcy and Lizzy than actually existed, one that would make a letter from Darcy not only unremarkable but (in the current dire circumstances) expected. Aunt Gardiner still imagines some serious relationship exists when she and Mr. G consent to Darcy's help with Lydia, as noted in her letter to Lizzy.

Don't think me angry, however, for I only mean to let you know that I had not imagined such enquiries to be necessary on your side. If you do not choose to understand me, forgive my impertinence. Your uncle is as much surprised as I am, and nothing but the belief of your being a party concerned would have allowed him to act as he has done.
...
Pray forgive me if I have been very presuming; or at least do not punish me so far as to exclude me from P. I shall never be quite happy till I have been all round the park. A low phaeton, with a nice little pair of ponies, would be the very thing. (Ch. 52)