Was the tests before and after swapping out your SIM done at the same time of day both times? If not, your results could be skewed. Verizon could have been experiencing network problems during that time too.
Other factors like server loads and excessive network traffic will affect your download speed as well. Even just being on another part of the building can effect signal strength and quality.
Other factors like server loads and excessive network traffic will affect your download speed as well. Even just being on another part of the building can effect signal strength and quality.