What you are passing into the variables from Workflow are the selected values, not the displayed labels.

In order to do any changes on the checkbox labels dynamically, you would need to utilize Javascript.  If you want to go that route, I'm happy to provide some guidance on how that could be done, I would just need to know if you are wanting to do this in the Classic Designer or the New Designer.

EDIT TO ADD - Here's the Javascript Code.  In both cases, the checkbox is field q1 and the single line field with the options is field q2, you'll need to update the Javascript accordingly to match your actual fields.

For the Classic Designer.  This requires your checkbox field to have more options that your custom options could be at a maximum.  It's going to update the labels and values on the fly, and hide the unused options. 

$(document).ready(function() {
  
  var checkboxValues = $('#q2 input').val().split(',');
  var checkboxCounter = 0; 
  $('#q1 .choice label').each(function() { 
    $(this).text(checkboxValues[checkboxCounter]);
    $(this).parent().find('input').attr('value', (checkboxValues[checkboxCounter]).replace(/ /g,"_"));
    if(checkboxValues[checkboxCounter] == undefined) { 
      $(this).parent().hide(); 
    }
    checkboxCounter++;
  });
  
});

For the New Designer (this requires Forms 11 Update 5 - which is 11.0.2311 - or newer).  In this case, it doesn't matter how many options your checkbox has, because they will all be replaced. 

var checkboxValues = LFForm.getFieldValues({fieldId: 2}).split(',');
LFForm.changeFieldOptions( { fieldId: 1 }, checkboxValues, "replace" );