A fluorescent bulb requires a big amount of voltage to fire up. By wiring the single bulb in series to a ballast you are doubling the startup voltage. The bulb can't handle that. When you overdrive a bulb with a proper overdrive ballast the startup voltage is not doubled and everything stays within spec. Also the bulb has to allow being overdriven in the first place or life is reduced significantly. Let's use T5 VHO as an example. When you use an icecap 660 which is a VHO ballast the startup voltage is still within T5 spec.