Possible reasons for short Phal spikes?

[Lacie]

I have been growing orchids about 5 years now, and have 6 Phals (two species, the rest hybrids, mostly NoIDs). A few of my larger Phals the last few times they have bloomed have put out very short spikes. They still spike regularly and bloom fine, but I was wondering if people had thoughts about what causes the short spikes? My guess is it is either from under-fertilizing (I don't fertilize often, maybe once every few months), or low humidity (I'm a windowsill grower; I have humidity trays and sometimes mist, but that's it. My plants all seems to do fine anyway so I've never worried too much about humidity).

Obviously not a major problem, but curious if people know why this happens. I just think longer spikes look more impressive

I've attached pictures of one that has had this issue. The first photo is of it currently in bud (spike is roughly 8"), the second is from the last time it bloomed. I have another plant that did the same think, the last time it bloomed the spike was maybe 6" and only produced a few flowers, but I couldn't find a picture.