  We present a new Keck/NIRSPEC observation of metastable helium absorption
from the upper atmosphere of HD 189733b, a hot Jupiter orbiting a nearby
moderately active star. We measure an average helium transit depth of $0.420
\pm 0.013$% integrated over the [-20, 20] km/s velocity range. Comparing this
measurement to eight previously published transit observations with different
instruments, we find that our depth is 32% (9$\sigma$) lower than the average
of the three CARMENES transits, but only 16% (4.4$\sigma$) lower than the
average of the five GIANO transits. We perform 1D hydrodynamical simulations of
the outflow, and find that XUV variability on the order of 33%--common for this
star--can change the helium absorption depth by 60%. We conclude that changes
in stellar XUV flux can explain the observational variability in helium
absorption. 3D models are necessary to explore other sources of variability,
such as shear instability and changing stellar wind conditions.
