According to the Unicode Standard, U+03C6 should use the loopy form of the Greek letter Phi and U+03D5 should use the straight form of the letter. Source Code Pro uses the straight form for U+03C6 and the loopy form for U+03D5.

Excerpt from the relevant paragraphs in the Unicode Standard:

For mathematical and technical use, the straight form of the small phi is an important symbol and needs to be consistently distinguishable from the loopy form. […]

[…] fonts that also intend to support technical use of the Greek letters should use the loopy form [for U+03C6] to ensure appropriate contrast with the straight form used for U+03D5.

See Unicode 8.0.0, Chapter 7.2, “Representative Glyphs for Greek Phi”