When you write a query letter, you’re trying to condense a lot of information into a short amount of space. This often involves using more complicated sentence structures. Which, in turn, often leads to clunky sentence structures.
If you think this a problem for you, I recommend finding a copy of William Strunk Jr. and E.B. White’s Elements of Style. This is the book used by just about every writing and editing class I’ve been to at BYU. And the advice is phenomenal.
The tips on forming smooth sentences will not only help your query, but your entire manuscript. So get your hands on a copy if you can. It’s a great resource!