One of the main reasons why carpets smell after a deep clean is the underlay, that’s basically because this part of the carpet padding is not exposed to direct light or air, which means it can take a lot more time to dry properly.
The excess moisture can lead to mould, causing a mildew smell after the carpet cleaning process. This type of fungi is responsible for the musky smell coming out of your living room floor covering.
This can happen more frequently if the carpet is located in a room with less air movement and sunlight, particularly a house that could have a south facing back garden – meaning the living room which is usually found at the north side of the house won’t see much daylight than rooms near the south.