The "POP.B" instruction is equivalent to "MOB.B @R1+".
In general, with the auto-increment addressing mode, registers are incremented by 2 for word instructions and by one for byte instructions. However, there is one exception to this rule: the R1(SP) register. Whatever the instruction type is (i.e. word or byte), R1 will always be incremented by two when addressed with the autoincrement mode.
The current implementation of the openMSP430 doesn't implement this.
Bug fixed with SVN revision 130.