Documentation says the convert_value function has a parameter that tells if the lookup should be strict of not:
/**

• Find the closest matching numerical input values in a list an replace it by the
• corresponding value within another list
• @author Johannes Weberhofer, 2013
• @param numeric $fValueToReplace
• @param numeric $iStrict - 1 for exact matches only otherwise interpolation the
• closest value should be returned
• @param string $sTranslateFromList - comma seperated list of numeric values to translate from
• @param string $sTranslateToList - comma seperated list of numeric values to translate to
• @return numeric
  */

This seems to work ok for javascript (in page) evaluation, but not for backend evaluation.