Agents are on commission as are you. Her question isn't entirely fair because it is generally assumed if you have no prior commitments with an agent or publisher, you may already have put out dozens of queries.
However, since she asked, I would give a brief, complete, and honest answer without using specific brokerage names.
On Friday, June 10, 2016, you requested a partial manuscript to HEARTLESS. Attached is that manuscript. You also wished to know if there were other agents having requested and received submissions. As of Monday, June 13, 2016, I have communicated directly with four agencies including yourself and submitted one complete manuscript. That manuscript was sent electronically Monday, June 6, 2016.
Thank you for your time and interest. I look forward to hearing from you.,
I would put no more no less.
As to the logic, she's probably just trying to plan her month. An agent who's just decided to read your book needs to reserve a full day of time for, say, every 40,000 words. An agent listed anywhere online as accepting submissions receives at least 15 queries a day.* Try not to let her logic psych you out; it is irrelevant in consideration of supply and demand. If you've gotten this far, congratulations, you've done better than 99%+ of aspiring authors.
- This information was taken from Jane Friedman's blog on publishing. She is an agent of the finest quality.