Actually, interior paints are a different formula from exterior paints so the tinting formula is likely slightly different. The refractory index may also be different which can also appear to change the color a bit. So technically, the clerk may have been right when he said he couldn't tint it.
Having said that, an experienced tinter would have been able to make any small adjustments to get it really close...however you generally don't find them at Home Depot. Which is why I prefer not to use Behr paint. It's the quality of the support staff, not so much the paint itself (although I do prefer some other brands better).
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