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-ode <sub>(or, to be more exact, the 1300D would deactivate AF)</sub> :

> [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).

<sup>[See p. 100 in your camera's manual](http://gdlp01.c-wss.com/gds/2/0300023332/02/EOS_1300D_Instruction_Manual_EN.pdf)</sup>

----

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`.