[Sever]

I found a list of the hydropathy index of the 20 AAs, but I’m a tad confused. Negative values mean that they are hydrophilic and positive values mean that they are hydrophobic. With that being said, why do some sources refer to P and G as hydrophilic and C as hydrophobic.

I get G to a degree which is the least hydrophobic given the value of -0.4 and I could reasonably wrap my head around P and C as well if they were also close to 0 but in reality they both have values of -1.6 and 2.5 respectively.

So why, based on these values, are P and G considered hydrophilic and C hydrophilic by some sources?

[Babcock_Hall]

I do not know the answer to your question, but Jack Kyte's books might be a good place to look.  The definition of hydropathy is not quite the same as hydrophobicity or hydrophilicity, and perhaps the answer lies somewhere in these details.