This is not a "headscratcher", it's something that honestly Just Bugs Me. When Bernard and Manny are pouring out the incredibly expensive Vin du Rosier, a little money ticker appears on the screen. Every time they pour some into their glasses, it goes up. When they drink it, it goes up again. When Bernard does a Spit Take, it goes up yet again. It reaches the value of the bottle when they pour the last drops into their glasses, even though they haven't finished drinking yet. There's no Fridge Logic — it's clearly pure Rule of Funny, but it still drives me crazy.
What exactly is the ethnicity of Manny? Why does this blond guy with obviously British parents have the very Italian last name of Bianco? Why aren't the parents' names mentioned? Is Manny a nickname, and does this make him Emmanuel Bianco? IMDb says the character was originally intended to be Jewish, which doesn't make things any clearer.
Ethnic soup mix. Which sort of makes sense, in London.
Worth noting that in the commentaries Dylan Moran says the character was meant to be Jewish, and confirms that Manny's short for Immanuel.
Bianco is Italian for "white", which is the opposite of "black". He has to have a foreign name because Manny White and Bernard Black is too obvious and unlikely.
Simple: his mother is Jewish and his father is Italian.
The Italian side of that argument is Jossed by the fact that we see his parents and they are both British. Doesn't mean that one or both of them might not be Jewish, though, or have Jewish or Italian forebears. There's even a bit of (probably unintended Fridge Logic in their casting: Anette Crosby is Scottish, and Scotland has a high Italian-immigrant population. For comparison, see Armando Iannucci, a Scottish man with the most Italian name you can imagine.
Britain, despite what certain tabloids would have you believe, has been a melting pot of immigrants for centuries. There is no combination of traditionally ethnic names that is not beyond the realm of possibility or believability.
In the episode 'Grapes of Wrath', just after calling the cleaning agency, Manny spots a slice of toast stuck to the ceiling - the apparent reason for this is shown towards the end when Bernard launches another slice upwards after tasting it BUT the one that Manny saw was not only jam side down but also didn't have a bite missing from it!
Bernard is an irrational guy, maybe he prepared the toast, putting jam on both sides, and then decided he didn't want it without eating any. So he threw it at the ceiling! Don't act like you've never done the same...