The -10dB shown in the second image of the polar pattern is a rather generous 'average' of some sort. This pattern is typical for a super-cardioid mic & it will display similar characteristics at all frequencies, but in the absence of specific frequency maps in the second image, you must assume they're using averaging - in this case to make the product look a lot better than it is, unfortunately.
If you look at the top image, you see the rear rejection up to about 2k is a reasonable -20dB, but by the time you get to 5k it's almost at parity [perhaps -4dB]. This means that at higher frequencies the mic will behave more like a figure-of-8 pickup than a super-cardioid.
I found an article - Cobalt Audio - SM58 and Beta 58A – Comparison - showing the polar patterns of two well-known, industry standard mics, the Shure SM58 & Beta 58 [cardioid & super-cardioid respectively].
Note how they show pickup details at several frequencies rather than just an overall average, but that even good mics have less rear rejection at some frequencies than others. Because this can really never be 'perfect', these patterns are often tailored to specific use-cases, or are a 'best fit' compromise. In this instance, though, never worse than -10dB.