| This look more like programmed start ballast (most electronic starters by their behavior belong here, even when the circuit and separate ballast description say "preheat"): first preheat to visible glow (so significantly higher temperature then needed) and only after some time (when the filament temperature already settled) the ignition.
RS ballasts simply apply across the tube voltage insufficient for cold electrode ignition, but sufficient for hot electrode ignition and let the electrodes heat up. So the lamp ignite at the moment, when the electrode temperature reach exactly the level, when the applied voltage is exactly enough to ignite them. So this happen way sooner, then the filament temperature settle. And as each electrode is different, each of them reach this point at different time, so the turn ON look more like gradual light-up (first the lamp ignite dimly in rectification mode, a bit later to full light). Some electronic starters belong here, as they provide electrode heating and at the same time generate starting pulses - so the lamp ignition moment is, when electrodes just reach required temperature.
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