So what was different on this occasion ?
A few thoughts
Maybe the runway profile, including the ramping, during the period concerned didn't allow water to quickly drain off the ungrooved surface. strong crosswinds with rain may have had similar effect.
The re-profiling of the runway which required base course to be laid separately which other airport resufacing projects may not require.
This re-profiling and consequent areas of ungrooved base course shown in Figure 5 of the report seem to be spread across a lot of the surface which may not have helped.
The wet weather preventing an accurate friction assessment to be carried out using self wetting device (always a problem during winter months).
Unfamiliarity with friction runs and reporting under such conditions.