The Weather Prediction Center (WPC) updates the fronts and pressure information on the Prog Charts at different times throughout the day (see http://www.wpc.ncep.noaa.gov/basicwx/about_bawx.shtml). For example, at approximately 14Z, the 6 and 12 hr forecasts are updated, but the 18 and 24 hour Prog Charts don't get updated until about 1630Z. So for a period of time (between 14Z and 1630Z), you'll see the valid times for the 6 and 12 hr forecasts be the same as the valid times for the previous 18 and 24 hour forecasts (that were updated about 0430Z). In ForeFlight, the Imagery isn't designed to "omit" charts - it just displays the image located at the static URL where we pull the forecast from NOAA.
Note also that the precipitation shown on the Prog Charts isn't created by the WPC. It comes from the National Weather Service (NWS) local weather forecast offices. This part of the Prog Charts gets updated every three hours. Therefore, you may see issuance times get updated more frequently than the times shown in the WPC help page above.