  We study the late-time evolution of the central regions of two Milky Way-like
simulations of galaxies formed in a cosmological context, one hosting a fast
bar and the other a slow one. We find that bar length, R_b, measurements
fluctuate on a dynamical timescale by up to 100%, depending on the spiral
structure strength and measurement threshold. The bar amplitude oscillates by
about 15%, correlating with R_b. The Tremaine-Weinberg-method estimates of the
bars' instantaneous pattern speeds show variations around the mean of up to
~20%, typically anti-correlating with the bar length and strength. Through
power spectrum analyses, we establish that these bar pulsations, with a period
in the range ~60-200 Myr, result from its interaction with multiple spiral
modes, which are coupled with the bar. Because of the presence of odd spiral
modes, the two bar halves typically do not connect at exactly the same time to
a spiral arm, and their individual lengths can be significantly offset. We
estimated that in about 50% of bar measurements in Milky Way-mass external
galaxies, the bar lengths of SBab type galaxies are overestimated by ~15% and
those of SBbc types by ~55%. Consequently, bars longer than their corotation
radius reported in the literature, dubbed "ultra-fast bars", may simply
correspond to the largest biases. Given that the Scutum-Centaurus arm is likely
connected to the near half of the Milky Way bar, recent direct measurements may
be overestimating its length by 1-1.5 kpc, while its present pattern speed may
be 5-10 km/s/kpc smaller than its time-averaged value.
