[Zero Sum]

If I tell you someone is an thin as a rail, it could be an accurate statement, yet it lacks precision.

That's a simile: It's meant to express something comparatively, but it's not literally true. Although I'm sure most people would understand what you meant if you said that. ;)

[GoLightly]

That's a simile: It's meant to express something comparatively, but it's not literally true.

Something doesn't have to be literally true to be true. Similes, metaphors, personifications & the like are forms of expression. I could have left out the comparison. If I had just said, "the woman is thin", it could have been an accurate statement, yet it would be one that lacks precision.