...it is not what she expects. The General (IMO) reads this as disapproval and is insecure about it. So he tries to find out why. Notice how defensive he is about the domestic china when Catherine admires it; he wants her to know why he chose it, that domestic manufacture has grown even better since he bought his (practical) set.
An abbey! Yes, it was delightful to be really in an abbey! But she doubted...whether anything within her observation would have given her the consciousness. The furniture was in all the profusion and elegance of modern taste. The fireplace...was contracted to a Rumford.... The windows...were yet less what her fancy had portrayed. ... To an imagination which had hoped for the smallest divisions, and the heaviest stone–work, for painted glass, dirt, and cobwebs, the difference was very distressing.
The general, perceiving how her eye was employed, began to talk of the smallness of the room and simplicity of the furniture.... (Ch. 20)
The dining–parlour was a noble room... Catherine...saw little more than its spaciousness and the number of their attendants. Of the former, she spoke aloud her admiration; and the general, with a very gracious countenance, acknowledged that it was by no means an ill–sized room.... (Ch. 21)
The elegance of the breakfast set forced itself on Catherine’s notice when they were seated at table; and, luckily, it had been the general’s choice. He was enchanted by her approbation of his taste, confessed it to be neat and simple, thought it right to encourage the manufacture of his country; and for his part, to his uncritical palate, the tea was as well flavoured from the clay of Staffordshire, as from that of Dresden or Save. But this was quite an old set, purchased two years ago. The manufacture was much improved since that time.... (Ch. 22)
It appears that whenever Catherine notices something, the General wants to explain it, justify it, or otherwise guide her opinion to his way of thinking. I think he is defensive that her expectations are greater or her taste is more refined than his.