Incorrect historic Zimmerman calculation using Weather Underground PWS

I have checked the scales received from the weather service for the past days, if using Weather Underground PWS. Only the scale received for Yesterday corresponds to the calculation procedure defined by Zimmerman. Please look at the details below:

Weather Service Request

weather.opensprinkler.com/weather1.py?loc=49.47432,10.94365&wto=”h”:100,”t”:100,”r”:100,”bh”:65,”bt”:70,”br”:0,”provider”:”WU”,”pws”:”IFRTH239″,”key”:”my_api_key”

results in:

&scale=4&tz=56&sunrise=423&sunset=1156&eip=1469121255&rawData={“wp”:”WU”,”h”:83.53,”p”:0.34,”t”:67.7}&errCode=0&scales=[4,40,49,53,48,46]&ttl=81451159
The Weather Underground Weather History for IFRTH239 lists:

Date Temperature Average Humidity Average Precipitation Scale received -> calculated
September 21 19.8 °C 84 % 8,64 mm 4 -> 4 %
September 20 20,2 °C 82 % 0.00 mm 40 -> 77 %
September 19 18.4 °C 78 % 0.00 mm 49 -> 68 %
September 18 18.3 °C 80 % 0.00 mm 53 -> 66 %
September 17 14.3 °C 88 % 0.00 mm 48 -> 29 %
September 16 14.8 °C 80 % 0.00 mm 46 -> 40 %

The scales received for the days before yesterday seemed to be far too low calculated.
I have done a quick check for other PWS weather locations, and it looks like it is a general issue. I’m not sure anymore, but I have seen the wrong calculation for the first time having precipitation reported. I may be wrong in my findings, but please have a look at this issue.