Think about decimal numbers: does 53 have the same value as simply adding the values 5 and 3 ? Think about what the digit positions represent: does the 5 represent the value 5, or does its position tell us that it actually represents 50 ?
In the same way, the positions of the digits in 1000 and 0111 tells us what they represent. Just like with decimals, where the positions represent ...10000,1000,100,10,1... (powers of ten), for binary the positions represent ...256,128,64,32,16,8,4,2,1,... (powers of two).
So when you put 1000b in front of 0111b you did not take into account that the digit positions themselves encode information (the powers of two).
So concatenating 1000b (==8) and 0111b (==7) will not give the same as 10000111b (==135) , for exactly the same reason why 5 in 53 represents a different value to the 5 in 50000: because the digit positions tell us information that we need to take into account in understanding the digit value.
