Double boot sizing

Hi,
Being relatively new to mountaineering boots, I had a question on double boot sizing: are they often vastly different than single boots?
I wear a 44 in Salewa Crows, wore a 44.5 in my old Hanwag frictions and am swimming in 43s in both Scarpa inverno and Koflach arctis expe. What gives? 

It helps to try on boots before you buy them.

Four different mfg, and four different lasts.

Both the Inverno and Koflach boots are sold in UK sizes. Size conversions between EU, UK, US is not an exact science either.

UK sizes in Invernos are about 0.5 to 1 size smaller than US men's street shoe size as a good starting point.

FrankPS wrote: It helps to try on boots before you buy them.

Which is exactly how I discovered that with koflach and scarpa.... Thanks man. They didn't have smaller than 43 where I was, and finding places to try on double boots is challenging. 

DeLa Cruce wrote:

Which is exactly how I discovered that with koflach and scarpa.... Thanks man. They didn't have smaller than 43 where I was, and finding places to try on double boots is challenging. 

Years ago,  I ordered many plastic double boots, tried them on, then sent them back because they didn't fit. I, also, had nowhere locally to try them on. Kind of a pain in the arse to have to repackage them and pay to ship them back. But, it was better than being stuck with boots that didn't fit.