Why some Philips HPI-T lamps, bulging their arctube?

[dor123]

I've noticed that some Philips HPI-T lamps, have bulged arctube, despite they are intended for horizontal position.
Why this is happening?

[AngryHorse]

Aside from the quartz ‘ball’ type of discharge tube, pretty much all halide lamps will tend to bulge if left in long enough.

[dor123]

As the HPI-T intended to be used horizontally. Even if the reason related to preventing sodium migration to the arctube seals, The HPI-T arc is very stable compared to most American probe-start MH lamps, so I don't find any reasons for the upper area of the HPI-T arctube to bulge.
Vertical operated MH lamps, have less likely for their arctube to bulge.
At one of my meetings with Ash, he brought a Philips HPI-T Plus 400W 7th generation, which had in arctube bulged despite it isn't blackened at all (See attachment).