The European hierarchy increases the number of channels by a factor of four at each higher level, but if you look at the gross bit rates for each level you will observe that it is actually significantly greater than four times the bit rate of the level below it.
This is because a certain amount of additional overhead is required in order to ensure that multiplexing and de-multiplexing can be carried out successfully.
The plesiochronous nature of the hierarchy (from the Greek plesio meaning near and chronos meaning time) means that bit streams from different sources are not guaranteed to have exactly the same timing, even though the nominal bit rate is the same.
This is due to the fact that each piece of exchange equipment uses its own internal clock for timing purposes.
Although these clocks are extremely accurate they are not synchronised, and there will be enough difference in the actual bit-rates of the tributaries feeding into a given multiplexing level to require some additional overhead in order to provide the necessary synchronisation.