Gaining a number of temporary hitpoints smaller than the cap of a more powerful existing spell no longer adds any amount.[edit source]
Per the example given in the page, if you have lost 10 hitpoints of the 15 total gained from Level 3 Armor of Agathys, then cast Flase Life at Level 1 for 7 hitpoints, these are no longer added. Casting a higher level spell will still overwrite the previous one, if it's total exceeds the original.