This is what is known as banding.
The mortar used on that day was different.
It looks like it could be a weaker mix which is potentially serious.
'Jonny jet wash' couldn't sort it that's for sure!
The explanation of laying bricks in wet weather is ridiculous, as no bricklayer lays bricks in the rain.
It could have rained after the bricks were laid, or the bricks were damp when laid, but that would have caused efflorescence on both bricks and mortar. This is a white powder from salts leaching out of the wall as it dries.
You should name the housebuilder! The quality looks like Persimmon to me.
Write to the housebuilder and suggest it is a different mix and ask that a sample to tested at an independent laboratory. If the mortar is durable (correct mix) , then I would suggest the joints are tinted by a professional "brick doctor" as any repointing will also be of a different colour unless the whole elevation is re pointed.