This shouldn't be surprising at all.

Covid is something like 30k nucleotides and has 11 protein-coding genes. The spike protein used in one of the vaccines is 4,284 (Pfizer), 4101 (Moderna) nucleotides out of that 30k. This is about 14% of the total genome.

Epitopes are what your body identifies to neutralize pathogens. It stands to reason that an interaction with a more diverse and comprehensive pathogen will result in more immune epitopes being generated.

The fidelity (accuracy) of RNA polymerases during replication is about every 10,000 to 1,000,000 nucleotides.

The estimated number of virions produced during an infection is 10⁹ – 10¹¹.

So a high estimate of 1 error every 10k nucleotides gives 3 mutation per virion, while the low estimate gives 1 mutation for every 33 virions produced.

Going from that 10⁹ number of virions produced in an infection gives anywhere from 30 million to 3 billion mutated virions from a single infection!

So now imagine you inhaled some droplets, they will have numerous variations that your body will adapt to, while the vaccine has one single form of just one protein from SARS-COV-2, and it's a leaky vaccine that is at best only temporarily effective at reducing symptoms.

Viral loads of breakthrough Delta variant infection cases were 251 times higher than those of cases infected with old strains detected between March-April 2020.

Neutralizing antibody levels after vaccination and at diagnosis of the cases were lower than those in the matched uninfected controls. There was no correlation

https://en.wikipedia.org/wiki/Severe_acute_respiratory_syndrome_coronavirus_2#Genome

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7293463/

https://en.wikipedia.org/wiki/Pfizer%E2%80%93BioNTech_COVID-19_vaccine#Sequence

https://en.wikipedia.org/wiki/Epitope