Um... there is a reason, not very major though.
Enthalpy of formation of BaCl2 is -859
However, the reaction
BaCl2(s) --> Ba(s) + Cl2(g)
is actually the reverse of the reaction for formation
(for formation it goes like Ba(s) + Cl2(g)-->BaCl2(s))
Therefore you reverse the sign, and the enthalpy for the reaction as they put it is +859
Hope this helps.
selkie