With 26 first priority holes at 500m each that's 13,000 metres. The initial drill plan expected to drill 10,000 metres which included some deep holes (up to 1000m). This would mean many of the first priority holes would have been shorter than 500m or not all first priority holes were part of the initial drill plan.

Now that the drill plan is 20,000 m this could mean many of those initial holes will be drilled deeper because of good results at depths down to 500m, or they are moving on to second priority targets looking for results closer to the surface.

All just a guess unless you have an eagle eye view of what's going on down there on site.