Yes, the odd-one-out in my photo is the one on the far right, as Ray guessed, and for the reason that Cathy gave. But Tigerchips' reasoning, wondering about the effect of the small bowl and the twist (which is actually a mixed air twist and an opaque white thread), shows just the sort of thinking that is also necessary for a fuller analysis.
The smallest item, with the "stepped" foot happens to have the highest weight-height ratio, and by a very large degree! This may be because the glass has a higher lead content than the others or it could be because the foot is formed from "thick loops" which in total may be just as heavy as the foot and stem together of some other items.
Anyway, what I should point out is that my general weight-to-height ratio idea is only a very rough way to assess things. For an accurate analysis of this type it's the Specific Gravity that needs to be measured. And even then, the visual features probably count for more towards a final decision.
But, taking a bit of a ramble for a moment ... When we read about "light weight" being a "reason for suspicion", how can we know what this means unless there are known genuine items to compare with? And can we make any proper assessment of this type when, for instance, attending an auction viewing or examining items from a dealer's display? How many of us go out and about with a set of scales and a tape measure (or perhaps even a bucket of water and a thermometer, for Specific Gravity measures)?
Although I am happy enough that the other six pieces in my photo are 18th century items, I don't actually have guaranteed proof of that!