Some areas used to be strips of water / land, but too much peat was dug out, storms washed the remaining land strips into the water beside it, creating a regular lake. For example some lakes in Giethoorn area were formed like this.

Like most lakes in the NL these tend to be shallow (~1.5-2m depth is common, even the biggest lake -Ijsselmeer- is only about 5..6m in deepest areas).