Not sure if it's just me, but the website seems to be a bit buggy.
When I select a unit, then select another unit to compare DPS, the new unit's DPS does not show until I either click the squad again or click one of their weapons to compare it to- After writing it out I realized that's probably a feature to let you compare upgrade weapons, I approve!
What most certainly is not a feature though is that it seems to only be able to display weapon names from the currently selected unit
only. Randomly, or if a unit is selected that has no other attacks, it'll display the second line as "undefined".
Team weapons often cannot be compared, the stats from the previous weapon are simply removed when the new one is selected, for example when attempting to compare the DShK and M2, only one will be displayed at a time. It's heavily inconsistent, I haven't noted any particular pattern for why this happens; it randomly affects most MG's, Mortars, and AT Guns I've tried.