The Student Room Group

Is Calvin Klein's sizing off?

So I measured myself and calculated that I'm a 34b (like 2 years ago) and every kind of bra I have bought since then even from different shops that are a 34b have fit me perfectly but then there was a sale at Calvin Klein so I bought a balconette bra ( I have never worn this before, usually I wear t-shirt bras, demi bras or sports bras) and when it came I tried it on and it did fit but there was a bit of a gap near the top of the bra so I was questioning why? Is this just Calvin Klein or am I not a 34b? But every time I get measured it always says I'm a 34b and all my other bras which are a 34b still fit perfectly so I'm confused????