There is a difference between the noun לחם and the noun חלה (bread): The noun לחם includes anything that is food for a human, while the noun חלה specifies the form of the bread and its round shape; the root חל teaches about the roundness. And any place where it is writtenחלת לחם , it means round bread.

And there is another difference, in that the noun חלה can also be before it is baked but after it is made with its round shape.

And when it is written על חלת לחם חמץ (with cakes of leavened bread)⁶⁶⁹, it is impossible to interpret it to mean לחם חלה (round bread), since the noun חלה precedes חמץ. With this, we are forced to conclude that the noun לחם modifies the noun חלה, and it means to say that it needs the חלה to be לחם, meaning baked in an oven.

footnotes: ⁶⁶⁹ Leviticus 7:13.