Kiali UI uses a component to automate scales the Y-Axis with the metrics.
Code can be seen here:
https://github.com/kiali/kiali-ui/blob/master/src/components/Metrics/MetricsChartBase.tsx#L78
Basically what you are seeing is some units are automatically calculated in the way:
1B = one byte
900 mB = 900 mili-bytes (that's a synthetic metric).
Obviously there are units that are not continuous like bytes and it doesn't make sense more than to show the scale relative.
Which version are you running, last one based on 0.17 ?
I hope this helps.