What does 'Slip' mean?

Chapter 2: The Media and Property Prices

"... themselves as in the case of the following headlines from the BBC: 1st March 2005: "House prices show slight increase" 4th March 2005: "UK house prices Slip in February" 31st March 2005: "House prices dip worst in a decade" 5th April 2005: "House prices stage rise in March" and so..."

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Chapter 20: What a Property Survey Really Means

"...If we take London as an example, the fact is that most of the North is built on clay and most of the South is built on marsh. This means in the North properties tend to Slip around while in the South they tend to sink. It's a historical problem which dates back thousands of years to a time when people were more interested in living beside a river at a shallow point than somewhere that would be suitable for four or five storey buildings. ..."