There are two main components to semen, in short: stuff from the prostate, and stuff from the seminal vesicles. The prostate stuff is very thick, the other stuff not so much. Men tend to produce different proportions of it, so tend to have slightly different consistency. But in my experience, the secretions from the prostate build up slowly over time, while that from the seminal vesicles build up quickly during sex/masturbation. The longer you edge, the more volume of the semen will be made from the thinner stuff, which results in more watery consistency overall. If I jerk off as fast as I can in five minutes, it'll be just some fat thick globs dribbling out; if I do it for a couple hours, it's not only a lot more semen, but it's extremely watery and messy and gets everywhere.
This is also why guys getting fucked or using a toy, if they have a hands-free orgasm, is usually just some thick dribblings, not like normal spurts. It's just what's in the prostate being "milked" out.