Dai ca. To my understanding that is because the compiler is lazi. (Someone told me this but I don't know if it's true or not. lolz)
This is because size of this particular data structure is the result of combining the size of it individual members. Plus wasted bytes used by the compiler for alignment purposes if and when the data of the individual members is not of equal size. Like if you try above with just char, int, and float, it will be equal to 12. The compiler will put 3 wasted byte in after char so it can do 4, 8, and 12. O_o I guess it's easier to count in multiples of bytes than not. O_O
The compiler will add bytes so that the size of the structure is a multiple of a certain number of bits. On x86 it's 32, on x64 it's 64.
char array (10 bytes)
int (4 bytes)
float (4 bytes)
2 bytes of padding