Trying to understand how the same secret key end up being calculated between Alice and Bob

Question

This is basically a duplicate of this question which was not answered very clearly. From the documentation, these are the calculations that happen for Alice to compute the shared secret key:

DH1 = DH(IKA, SPKB)
DH2 = DH(EKA, IKB)
DH3 = DH(EKA, SPKB)
DH4 = DH(EKA, OPKB)
SK = KDF(DH1 || DH2 || DH3 || DH4)

If I understand correctly,

IKA -> Alice's private identity key

SPKb -> Bob's public signed prekey

IKB -> Bob's public identity key

EKA -> Alice's public ephermal key

OPKB -> Bob's public one time prekey

From the documentation: "Bob retrieves Alice's identity key and ephemeral key from the message. Bob also loads his identity private key, and the private key(s) corresponding to whichever signed prekey and one-time prekey (if any) Alice used. Using these keys, Bob repeats the DH and KDF calculations from the previous section to derive SK, and then deletes the DH values."

I interpreted this as bob doing the same computations above with following values:

IKA -> Alice's public identity key

SPKB -> Bob's private signed prekey

IKB -> Bob's private identity key

EKA -> Alice's public ephermal key

OPKB -> Bob's private one time prekey

What I don't understand is how the same secret shared key end up being computed even though for DH1, DH2, DH3, DH4 different values are used for the keys. I am probably missing some property of the DH function along with how public keys are generated from private keys that makes this possible. Or my whole understanding of this is incorrect. Any help would be appreciated thank you!