  UV and optical surveys are essential to gain insight into the processes
driving galaxy formation and evolution. The rest-frame UV emission is key to
measure the cosmic SFR. However, UV light is strongly reddened by dust. In
starburst galaxies, the UV colour and the attenuation are linked, allowing to
correct for dust extinction. Unfortunately, evidence has been accumulating that
the relation between UV colour and attenuation is different for normal
star-forming galaxies when compared to starburst galaxies. It is still not
understood why star-forming galaxies deviate from the UV colour-attenuation
relation of starburst galaxies. Previous work and models hint that the role of
the shape of the attenuation curve and the age of stellar populations have an
important role. In this paper we aim at understanding the fundamental reasons
to explain this deviation. We have used the CIGALE SED fitting code to model
the far UV to the far IR emission of a set of 7 reasonably face-on spiral
galaxies from the HRS. We have explored the influence of a wide range of
physical parameters to quantify their influence and impact on the accurate
determination of the attenuation from the UV colour, and why normal galaxies do
not follow the same relation as starburst galaxies. We have found that the
deviation can be best explained by intrinsic UV colour differences between
different regions in galaxies. Variations in the shape of the attenuation curve
can also play a secondary role. Standard age estimators of the stellar
populations prove to be poor predictors of the intrinsic UV colour. These
results are also retrieved on a sample of 58 galaxies when considering their
integrated fluxes. When correcting the emission of normal star-forming galaxies
for the attenuation, it is crucial to take into account possible variations in
the intrinsic UV colour as well as variations of the shape of the attenuation
curve.
