For practical purposes the 11 volt versus 11.5 volt rating makes no appreciable difference. In my mind it is much like a sylvania light bulb rated 60watts at 115 volts ac and a general electric bulb rated 60 watts at120 volts. In theory the lower voltage bulb will burn less than five percent brighter. Manufacturing tollerance variations will tend to negate any real difference to start with. Even if you got bosh plugs from two different production runs you might have a greater spread. It is a good test to see if one brand tends to last longer than another if done objectivly perhaps. Since you only have the one differemt manufactures plug a random efffect is possible. In general though if the beru outlasts two or more bosh plugs I would buy beru from now on or at least try a set when your present plugs are gone eventually. Again though nothing is sure as a random production effect might overshadow consistancy hence forcastibility of the beru or the bosh for that matter. Maybe glow plugs are a bit of a crap shoot although the cheap brands have a poor reputation. Brands like autolight, champion etc should be avoided basically by others experiences of very short apparent lifespans on occasion. There is absolutly no potential problem with your mix though that occurs to me.