i was a injection moulding machine setter using rubber and we made a lot of squeegee rubber for some well known companys .
the rubber normally used in this production is the left overs from bigger products just so it gets used up rather than wasted .
the rubber compound is in diffrent hardness for a soft blade a 25 epdm rubber is the normal 1 to be used , for hard it was 50 epdm being used .
but there was times when both the 25 & 50 epdm rubber would be fed into the machince at the same time so the quality isnt as good .
some blades are diped in a silacone dip before being packed and this leaves the rubber looking shiney and can cause a film to be left on the glass until the silacone wears off the blade , you can use vineger to remove this just wipe with a cloth .
imo the best quality rubber that is being used on blades is the wagtail rubber as this can not be mixed with any other types as its a red rubber and theres only 1 hardness of red rubber .
other methods of production of a product can allso make a slight diffrance , copression will give a better edge on the blade and feel slightly harder , extrusion will 99% of the time give same quality .