I imagine there's this sort of unspoken contest between omegas. It's common for them to brag about certain aspects of their alphas to try to show up other omega (a bit like stereotypical housewives).
"My alpha just bought us that new tv set we've been eyeing."
"My alpha's so virile, we just found out we're having twins!"
When Sherlock and John get together, Sherlock is so proud of John that (even though he hates the idea of the competition and finds the other omegas tedious) he finally joins in but (in usual Sherlock fashion) chooses the oddest things to brag about.
"John produces [high but realistic for an alpha] CCs of semen when he ejaculates, even when I'm outside of heat."
"John's knot expands to [number] millimeters which is perfect considering [number + 2 or 3] is the number of millimeters he could expand to without actually damaging me."
"John's semen contains [high number of an alpha] [whatever measurement sperm counts are measured in]."
Every time he brags, people are shocked to silence because they aren't quite sure how to react. Sherlock takes this to mean he's won.
TL;DR - Omegas commonly brag about their alphas when they get together. Sherlock and John get together and Sherlock chooses slightly inappropriate (but very Sherlock-like) things to brag about. People are shocked to silence, Sherlock takes this to mean he's won.
Omega!Sherlock is proud of his Alpha!John
"My alpha just bought us that new tv set we've been eyeing."
"My alpha's so virile, we just found out we're having twins!"
When Sherlock and John get together, Sherlock is so proud of John that (even though he hates the idea of the competition and finds the other omegas tedious) he finally joins in but (in usual Sherlock fashion) chooses the oddest things to brag about.
"John produces [high but realistic for an alpha] CCs of semen when he ejaculates, even when I'm outside of heat."
"John's knot expands to [number] millimeters which is perfect considering [number + 2 or 3] is the number of millimeters he could expand to without actually damaging me."
"John's semen contains [high number of an alpha] [whatever measurement sperm counts are measured in]."
Every time he brags, people are shocked to silence because they aren't quite sure how to react. Sherlock takes this to mean he's won.
TL;DR - Omegas commonly brag about their alphas when they get together. Sherlock and John get together and Sherlock chooses slightly inappropriate (but very Sherlock-like) things to brag about. People are shocked to silence, Sherlock takes this to mean he's won.