I don't think this is true in general. Most patents I have read have had very detailed descriptions of the invention. In fact, it is to the patentee's benefit to be as detailed as possible in describing the invention. It will be harder for people to claim you didn't invent something if you have completely described it in your patent.

In contrast, what is actually claimed (that is, the bounds of the legal right to exclude that a patent grants), is typically made as broad as possible. Claims will be included that are both broad and narrow, so that if the patent is ever used in court, and the broad claims are knocked out by some new prior art, there are still narrower claims that can be asserted.

In general, the patentee should describe every version of the invention they have conceived of, in as much detail as possible, so as to support both broad and narrow claims. If the description is not specific, it is generally for lack of time or money on the part of the patentee or inventor, and it results in a weaker patent.