Essentially manufacturers only have to consider a dry runway, this is the basis of factored certificated performance in the flight manual. Wet performance is factored dry.
Using dry performance on a damp runway is an operational issue which is in EU-OPS 1.480(a)(4); “… those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain “effectively dry” braking action even when moisture is present.”
The UK CAA warned operators of the importance of using the correct performance in FODCOM 2009/03
“It is not sufficient for a runway to be considered, for performance purposes, as dry when it is wet solely on the basis that it is constructed with, for example, grooves or porous friction course pavement. Dry runway performance must only be used when the CAA has accepted in writing that the aeroplane can actually achieve the “effectively dry” braking action referred to in the EU-OPS definition.
…, there is currently no provision in the UK for notifying operators of runways having such surfaces, and the CAA is not aware of runways elsewhere that fully meet the “effectively dry” criterion.” (My bold)
The FODCOM has been superseded by CAP 789 ‘Requirements and guidance material for operators’, para 5.4.
“ a) Although a runway may have a grooved or porous surface, it may not be possible to demonstrate that it retains an 'effectively dry' braking action when wet. This may be because the type of surface is inherently not physically capable of retaining dry braking friction characteristics in the presence of sufficient moisture to be termed 'wet'. In other cases, it may be as a result of a surface reaching the end of its design life or that the recommended routine maintenance procedures have not been sufficiently effective … This is particularly relevant in the case of accumulation of rubber deposits in touchdown zones.
b) The effectiveness of aircraft anti-skid systems is especially sensitive to the presence of water on the runway. Aerodrome operators' Continuous Friction Measuring Equipment does not record a corresponding response to the presence of water, and thus operators may not rely on runway friction reports alone for demonstrating that a runway can retain 'effectively dry' braking action. Verification of the aircraft's braking performance capability in wet conditions is also required.
This would normally require support from the aircraft manufacturer.”
i.e. operators may not know or be able to verify how damp the runway surface is, nor quantify the maintenance condition of the runway; thus in order to plan for a safe landing, damp = wet.