These two types of tests often "see" things differently as well as "seeing" different things. It it pretty much routine to clarify a questionable suspicious finding. Regards ....
From my own personal experience, sometimes a mammogram just can't "see" your breasts well enough, like if you have dense breasts. Also, mammos can't really distinguish well enough between benign fibrocystic breast changes and something that might be suspicious, so for me I always have to have an ultrasound as well.
Others may have a better answer for you as well. Hope this helps!