Meters may read +/- 20% from the actual value of the blood
so 508 +/- 20% -> 406.4 to 609.6
and 578 +/-20% -> 462.4 to 693.6
Because these ranges overlap, the numbers may be considered equal.
And at these numbers, the specific number doesn't much matter - the glucose is too high and you'll be looking for ways to help reduce it - following your insulin guidelines, looking at food options, etc.