I work in a venue and we always notice this. Why? Well I suppose if you look at the group as a whole, the person wearing white always stands out. It's supposed to be like that.
I've seen hundreds and hundreds of weddings, line ups, group photo's- and as a society, we expect our eye to be drawn to the person wearing the big white dress. That's why people are there. Those 2 people are the centre of attention on that day. If someone else is wearing that colour, it draws the eye.
Now, on an individual level, does it matter if you wear that dress? Of course not. But on a group basis, you are attending an event where everyone is expecting to be looking at the person in white. It's a signal, I guess.
If you were being ungenerous, you could argue that the people who also wear white are going 'look at me!' If you are being generous, you could argue that they just didn't think about it. And to be honest, we've definitely see guests who have done this on purpose and we've also seen people who have literally turned up head to toe in white and not realised.
We've also watched the photographer discretely try to move these people so they don't draw the eye on in the pictures. Again, on a individual basis, does it matter? No. On a group basis, where couples have paid £1000's to have the perfect pictures, and there is someone wearing bright white and they aren't the bride, it does draw the eye.
And the bride might not give two hoots. But she might.
The dress is beautiful tho.