When the left ventricle is not satisfactory supplied with oxygen rich blood through the coronaries, the function of the ventricle may suffer and so the LVEF can become lower.
However:
The normal range of LVEF is 55-70%, so in both tests it came out normal. Also the test for EF is not very reliable and measurement outcomes sometimes vary, while the actual EF did not.
Unless your husband is not feeling well (shortness of breath etc), I woud not get too worried about this 55%