Dear student,
We suppose your confusion arises from the definition of magnification in case of simple microscopes, which says that it is the ratio of angle formed after and before using a lens.
When you remove the lens, it not the same angle ACB that you measure, but the one when the object is kept at the least distance of distinct vision.(D)
In the above case, the distance CB is smaller than D and thus it has actually resulted in magnification.
