I think most of the picking gets done right after the coins are dropped. Pennies (polished copper) are darker in colour than all other denominations (polished nickel) and so are the hardest to see on the ground (especially when discoloured). They are also the coins that people least care to pick up when they drop. We know that it usually takes a while for stuff to get buried out of sight and stay there. So, out of every however many coins that get dropped on the ground, I would expect that >80% of the ones that stay on the ground to get buried are pennies (and my finds are consistent with this).
I think that the presence of the pennies would indicate that other detectorists either haven't been there or didn't do a good job. How many people who post on this forum would pass up digging a coin just because it's probably a penny? If it sounds to me to even be similar to a coin, then the ground on top of it is about to get broken. Keep swinging and dig everything that hits cleanly. Then, when things stop hitting cleanly, start digging the dirty signals. Odds are that there is some silver somewhere nearby. You'll just have to dig a lot of pennies and pulltabs to get to it.