The last 600m will be through a tougher drilling formation,...

The last 600m will be through a tougher drilling formation, hence the longer time frame to get through it.
Rock Formation becomes more consolidated at the lower part of the well. They may also run some formation tests to check the lithology (ie sidewall cores, RFTs) to get an idea of what's down there. This all involves tripping pipe in and out of hole to run the required assemblies which all takes time. It's the interesting part of the well so they'll want to get as much information from it as possible.