In a lesson an answer attempt is always calculated as entirely correct or entirely incorrect. Depending on the settings of the site, all answers or just the first answer is used to calculate the score. By default all attempts are used in the calculation. Suppose the weight of the first question in a module makes up 10% of the total weight of the module. If you answer this question incorrectly, the progress bar will look like this:

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The colored part is completely red because all questions were answered incorrectly. When you answer the question again and now provides the correct answer, the chart will not change as only the first attempt is scored. If all attempts count, the progress changes to this diagram:

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The white part has remained the same size (you have not progressed in the module), while the red part has become smaller as only 50% of the answers is now incorrect.