It may be that your meter/metering technique/shutter/aperture/lens/thermometer/developer/developer dilution/agitation technique are cumulatively sufficiently different from the conditions used to determine the ISO speed of the film as to require you to meter at a different Exposure Index ("EI").

If using your meter at an EI of 320 and increasing the exposure by one stop worked for you, it would be worthwhile just using an EI of 160 instead.

WRT the shots you metered at EI 400 and were unhappy with, was they lacking in shadow detail, or were they low in contrast and highlight "sparkle"? If the former, your problem was with under-exposure. If the latter, your problem may be with under-development.