The same thing happens with what I consider the more 'traditional' 12x12 layouts.
In America they have 12 x 12 inches - while in NZ we have 30 x 30cm. We also have 12 x 12, because lots of stuff comes from the US, but, stuff from NZ is often 30 x 30cm.
30 x 30 cm is often refered to as 12 x 12, but it is not quite the same as 12 x 12 inches. (Have I confused you yet?!) 30 x 30 is slightly (about 5mm in both directions) smaller. This would only be a problem if you did 12 x 12 layouts, and then tried to put them in a 30x30 album. I know this because I tried to rearrange my layouts once, and discovered that several wouldn't fit in the nice new album I had bought especially. Darn it! I now make a point of trimming all my 12 x 12 inch layouts to 30 x 30cm, before I start the layout, just in case a rearrange is needed in the future.