Product review (5 stars): dual pack helps cut costs, keeps flavors from going bad with non OEM parts
To understand this review you must know a simple thing about injection molded plastic parts.  Virgin plastic pellets are always the best source material, they are heated up to melting then a plunger squirts a needed volume into a mold through a channel while forcing the air out a hole at the back.

Knock off the piece of plastic that was the input channel and the whole for air to escape and you have your part.

Those wasted pieces go via a vacuum tube off to the regrind system and are chipped up into, well, chips that resemble the virgin pelets used to make the part.  Only these chips are not virgin, they have been melted once. Good mfgrs make the best parts from all virgin material, but that is not cost effective.  NON-OEM (aftermarket) mfrgs take the regrind chips and mix in some percentage of regrind with virgin pelets to make a lower grade of plastic.  In the end it all works, but customers of those products may notice an off-taste to the water through the plastic filter?  Why?  Every time the plastic is melted oils come out to the surface, and eventually they make it into the water - after 20 gallons of washwater the aftermarket will taste the same as the OEM filter does with virgin plastic and just 1/2  - 1 gallon of washwater.

Some people can not even detect the taste, so for them aftermarket filters can lead to 40% savings.  Others can taste the difference (even the appearance is visually different from the regrind in plastics)

Buying a 2 pack saves some money - not 40% by any means, but since my dad and I use the same filter, and at about the same rate, we buy the factory OEM filters by the 2 pack for savings and good taste.

There are many more factors in doing injection molding I've left out - oils, release agents, FDA approved material being chosen overseas, etc - but it all plays into the part price and overall quality in the end.  It's a personal choice one must make.  I hope this helps - google "injection molding" regrind for more information about strength and other factors.