  This paper reveals the origin of variation in the magnitude and temperature
dependence of the normal state resistivity frequently observed in different
YBCO single crystal or thin film samples with the same $T_{c}$. We investigated
temperature dependence of resistivity in $YBa_{2}Cu_{3}O_{7-\delta}$ thin films
with 7- $\delta = 6.95$ and 6.90, which were subjected to annealing in argon at
400-420 K ($120-140^{o}C$). Before annealing these films exhibited a non-linear
$\rho_{ab}(T)$, with a flattening below 230 K, similar to $\rho_{b}(T)$ and
$\rho_{ab}(T)$ observed in untwinned and twinned YBCO crystals, respectively.
For all films the annealing causes an increase of resistivity and a
transformation of $\rho_{ab}(T)$ from a non-linear dependence towards a more
linear one (less flattening). In films with 7- $\delta = 6.90$ the increase of
resistivity is also associated with an increase in $T_{c}$. We proposed the
model that provides an explanation of these phenomena in terms of thermally
activated redistribution of residual O(5) oxygens in the chain-layer of YBCO.
Good agreement between the experimental data for $\rho_{ab}(t,T)$, where t is
the annealing time, and numerical calculations was obtained.
