On en-face OCT images this difference is easy to visualize.
In the retinal vessels the intraluminar content is hyperreflective. In the choroidal vessels the intraluminar content is hyperreflective.
Vessel caliber difference may be the key reason. Retinal Vessels have smaller caliber and while Choroid has larger vessels (OCT being able to resolve intraluminal content while not displaying the lumen of Retinal Vessels).
Nonetheless, it is much dependant on the depth level of the enface slab. If segmentation is from above the vessel, vessels would appear Hyper and if from below the Vessel they have displayed hypo and current OCT systems don't display enface segmentation from underneath the choroid.
Figure 7 B & E both show retinal vessels but segmented at different depth levels
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You may consider two additional factors:
1) blood absorption of NIR light is higher due to thicker medium (larger caliber) of choroidal vessels. It also depends on wavelength, i.e. 800nm OCT systems would have higher absorption than 1000nm one;
2) interference fringes pattern (and consequently OCT signal) may vanishes due to much higher blood flow speeds within the choroidal vessels than within cappilaries within the retina. However it depends on the A-scan/B-scan rate of your OCT/OCTA system. It is unlikely to reveal intraluminal signal from choroid vessels with an A-scan rate of 50-100kHz - you need much higher speeds. For more info, you may start e.g. from this publication doi: 10.1364/BOE.10.000050 and other ones cited inside.
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