This is by design, if a learner has more than one piece of evidence needing marking, the "Requires Attention" card will only show one item per learner on the dashboard. This means there may be additional items that require attention.
For example, your dashboard might display 4 learners, but when you delve into the "Requires Marking" section, you could see 6 pieces of evidence. This occurs because two of those learners have more than one item that needs to be marked.