Why do sentences that start with "guess" end with a question mark?

To me, sentences that start with "Guess" are in the imperative mood, thus, should end with a period:

• Guess who's coming to town.
• Guess what we had for dinner last night.

Why do a lot of publications put a question mark at the end of these sentences? I've been taught that you put a question mark only if the sentence is in the interrogative "mood". In these cases, we're not asking who's coming to town or what we had for dinner last night - we know who's coming and what we ate. We're telling that second person to guess.

If "Guess _ ?" is correct because it expects an answer just like a regular question, is it acceptable, then, to write:

• Tell me who's coming to town?
• Tell me what I had for dinner last night because I forgot?

"Tell __ ?" looks just as unsettling to me as "Guess _?".

Solution 1:

Because you have been taught an oversimplification.

Most English speakers would have no idea what you were talking about if you mentioned the "interrogative mood". People put a question mark on the end if it feels like a question.

Conversely, a polite order like

Would you sit down.

or a less polite one like

Will you sit down!

are often written without question marks, because although syntactically they have the form of questions, they are not in fact questions at all.

Solution 2:

There are two ways of using those sentences. One is the literal imperative, but the other is an informal form of the question "Can you guess....". The latter version is, in fact, often spoken with a rising inflection at the end, and is understood as an invitation/query rather than a command.

I'd say that the question mark is, in fact, an indication that the speaker intended the second usage.