I understand your confusion, but this appears to simply be an exception to the rule. The person could have just said, "they are soft inside" and not included the preposition at all :)

Hello Toru, Generally, we use at, in and on when we talk about the location of things. What do the prepostions in these three things tell us about the locations? "Meet Simon at the end of the road." "You left your glasses in the bathroom." "Is that a spider on the wall?" At a Point At tells us that the following noun is located at a specific point or location. It shows an exact position. "She's waiting at the entrance." "He's sitting on his chair at his desk." "I work at a bank." In an Enclosed Space In tells us the noun is in an enclosed space (surround or closed off on all sides). Basically, when something is inside something. In a box. In a room. In a country. On a Surface On tells us that the following noun is located on a surface. Use on when one thing is attatched to or touching somthing. On the table. On the floor. On the chair. What's the difference bewteen these two sentences? She's at the library. She's in the library. She's at the library. - the emphasis is on the her location and the type of place she has gone to. She's in the library. - the emphasis is on the type of building she is in. Both of these sentences are fine to answer the question, "Where is she?" Now decide which preposition needs to be used, and I will check your answers for you. 1 - I always keep some extra money __ my bag in case of emergencies. on at in 2 - I'll read it tonight ___ home. in on at 3 - Do you live ___ a house or an apartment? at in on 4 - Did you learn English ___ Malta? in on at 5 - She grew up ___ a farm. in at on 6 - I read about it ___ the newspaper. at in on 7 - He went for a swim ___ the river in at on 8 - The dog's sleeping ___ the carpet. in on at 9 - The information is ___ the top of the page. in at on 10 - Were you ___ the party too? on in at