I agree with others, the thought is mostly because the "peasant" considered the dining room very good due personal preferences.
For the sake of fun, I decided to do some rough calculations. Note that some of the values are just estimates and guesses since I haven't been able to determine everything involved yet in
this thread.
Walls:I make few assumptions in here. First of all, I assume the walls were constructed from stones that has material value of 1. Also, I haven't tested it out yet, but I also assume that the constructed tile has basic value V calculated by multiplying the material value (MV) by item value (IV). Since the assumed material value is 1 and rough stone has item value of 3, each wall tile has value of 3.
Next, there are 22 tiles of wall in total. This means the walls have total value of 22*3=66. There are 6 "gaps" (the tiles with doors). Since the current theory I have is that the one such tile decreases the value of room by certain amount, which is related to the quality of room at the end and before the GR is included, we can't calculate this immediately. I have also figured out that, if there is more than one tile considered as "gap", the number of such tiles required for additional penalty is calculated by the average dimension of the room. Since the room in question has average dimension of (7+9)/2=8, no additional penalty is calculated.
floor:I haven't actually tested out the case of floor fungi, but the general rule is that the floor tile (natural, not constructed) has value of the material value. Soil has material value of 1 (determined by material template), so the value of each floor tile is 5*7+6=41. Note that each tile which has door is counted as floor tile.
Furniture:So, what we know about the furniture? Six table, six throne and six doors. Two doors are masterwork, with quality modifier of 12. Now, I'm making an assumption that rest of the furniture has the average quality of Fine, which has quality modifier of 3.
Next, another assumption: each peace of furniture is made from material (stone) that has material value of 1. Door, throne and table have the item value of 10. This means the estimated total value of furniture is about (12*2+3*16)*10=(24+48)*10=72*10=720. This value also assumes that the value of each door do affect the value of the room, which I think it does even though I haven't actually proofed it yet with testing.
Total Value:The roof has no effect to the room value, which means the value before the gap penalty is 66+41+720=827, which translates to Decent Dining Room (Decent has minimum value requirement of 500 while Fine has 1000).
Now, the penalty caused by the doorways: So far we know that only the penalty of one tile is used due 6<8. Now, the next thing is to actually determine the penalty. The test of this section is still incomplete, so there is two possibilities; 5 or 45. This is calculated by formula which takes into account the value of the furniture used to create the room designation. I just assumed the value of that piece of furniture was 3*10=30 due the assumed average.
Now, in either case, the room is considered as Decent. After all, 827-5=822>500 and 827-45=782>500.
The point of all this? Just some fun calculations that proves that the personal status of the dwarf does affect what is considered legendary and what is not. After all, since the doors were the only masterwork piece of furniture, there shouldn't be anything else that boosts the "legendary opinion".