Most/some third-party lenses report better apertures than they actually have - e.g. with f/6.3, the Sigma 150-600mm f5-6.3 Contemporary DG OS HSM would no longer work in AF-mode (or, to be more exact, the 1300D would deactivate AF) :
[If] the maximum aperture becomes higher than f/5.6, AF shooting will not be possible (except in FlexiZone - Single and Live mode during Live View shooting).
See p. 100 in your camera's manual
As a workaround, such lenses often report 5.6 as maximum aperture - so AF will still work. Most of the time, it is somewhat slower than the AF of a real f/5.6-lens.
This does not explain the f/4.5 at 150mm, though. My guess would be that it is easier to shift the whole max-aperture-value down than to just "correct" it for values above f/5.6.
On a side-note: If the Sigma lens really was f/4.5-5.6, they would definitely sell it as such, as larger apertures usually sell better (and at a higher price).