I suggest you read about X-Axis here
. This help topic contains information and examples which will be useful to you as they mention a scenario like yours.
In just a few words - by setting AutoRange="true" you specified that you want the RadChart to automatically extract and set a range. The engine that sets the min and max values for the axis is guided by the input min and max values and the number of the ticks to be shown. Having in mind these things - some values may be rounded, which is what happens in your case.
The general solution in cases as yours is to set a lower TicksDistance as shown in the provided link. Another solution, of course, is to set the AutoRange to false and manually specify the min and max values.
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