Hello, well the phrase is a little idiomatic, meaning that it is not to be taken literally. However (at least to me as an English speaker!) i feel there is some sense... i will try and explain the 'logic'.
Firstly, EVERY. This is actually not necessary. "Now and then" is a valid phrase, and it still means 'sometimes'. The EVERY is a type of specifier/quantifier of sorts. For example we have 'every day', 'every other year' (every 2 years) etc.
NOW is here this moment. As in the present. Maybe not NOW as I speak, but a moment in the past or future.
THEN is a later/subsequent moment. (NOT NOW). Here in this odd phrase it means after the NOW (so a second separate occasion).
So 'logically' (perhaps!) we have something I do in moments and also in later moments. So more than once, 'sometimes'! But actually it is not as frequent as sometimes. It means 'occasionally'. Look at this phrase: "We sometimes eat at this hotel, and every now and then we also stay the night."