A slightly bluish, white star such as Vega, which has a B-V of 0.0, has the same V and B magnitude.
A random star not significantly suffering interstellar extinction is expected to have a B-V of about 0.6, meaning the V magnitude is brighter than B by 0.6m.
The discrepancy between B and V magnitudes depends also on the presence of interstellar extinction, which is generally strongest near the galactic equator due to the flattened system of dark molecular clouds. And NGC 6910 is almost exactly on the glacic equator, and moreover lies in a region of quite notable fairly nearby dark clouds which make up the Great Rift.
As noted, the mean B-V for the cluster is a little larger than 1 magnitude, which for such a young and hence bluish object implies that this color excess is due almost entirely to extinction and reddening. This cluster is comparatively nearby. As we look to ever fainter stars in the field, some will most likely be even more distant non-members, and hence probably even more strongly dimmed by the greater optical depth of interstellar dust. It could be the case that some such distant stars could be suffering a color excess of 2, or 3, or 5 magnitudes, if not more. And so a B magnitude will be fainter than a V magnitude by that amount.
Incidentally, a color excess of 1 magnitude is a total visual extinction of about 3.1 magnitudes. In the absence of the intervening dusty clouds, NGC 6910 would otherwise be a bit more than 3 magnitudes brighter.