Typically there are only 3-4 crossovers per chromosome. This means that large chunks of DNA get copied together, and as such the assortment of every base is not as simple as assuming each has an independent chance of ending up in a gamete. Rather, nearby bases are extremely likely to go into the same gamete.
This is why it's quite possible to get gametes that are similar in quantities much higher than the "expected" 50%. Instead of being 3 billion items randomly assorted, it's roughly 22 chromosomes x 4-5 chunks = 100 of so chunks of DNA, albeit split at random locations.
This simply spreads out the "normal" distribution. 50% is still the expected similarity of any two gametes, but it's not surprising for any two to be much more similar, or much less similar.
This is why it's quite possible to get gametes that are similar in quantities much higher than the "expected" 50%. Instead of being 3 billion items randomly assorted, it's roughly 22 chromosomes x 4-5 chunks = 100 of so chunks of DNA, albeit split at random locations.
This simply spreads out the "normal" distribution. 50% is still the expected similarity of any two gametes, but it's not surprising for any two to be much more similar, or much less similar.