we both get the same buffs .....
If you guys are in different parties it's hard to say unless you were monitoring very closely yourself.
if bards are rotating : a common problem is one bard is a lot more well geared than the other, so the song durations are out of sync. With good song duration gear bard can have SV JA songs for ~7:40mins. With carn obviously more. A crappy bard would have a song duration of around 5mins.
So if the bad bard sang marches and the good bard sang minuets and the bad bard started on your march - unless they communicated (usually doesn't), you probably lost marches after ~5mins, and since the bad bard was in the other party, he might've went up and resang (even if he didn't his marches will last slightly longer over there due to being sang later). the better geared bard might not have noticed and thought - i still have 2 mins before i have to sing. Meaning you hit a lot slower, naturally losing dps.
If bards aren't rotating, you might've had weaker songs depending on what the instruments the bard in your pt has. Even if they are both ghorn 99 harp99 there's no guarantee that your bard might not have overwritten the harp songs with ghorn. Don't forget bard AF3 also gives stats boost. So how many pieces they are wearing when they buff also affect your attributes
There's also corsair, if you had any in your party, you might've had weaker buffs simply by getting less lucky numbers.
So unless you have been monitoring really closely and can say that your buffs are off equal to the point where your corsairs rolled the same numbers, otherwise i'd say it's an unfair test.