One thing to understand is that the Jewish calendar was not calculated until at least 358, possibly later. It's possible that the modern calculation didn't start until the 800s. Part of the ante-Nicene problem, apparently, was that different communities of foreign Jews didn't follow the same rules about determining Passover; also, there is pretty good evidence that some of them followed rules which ensured that it always fell in March, which would have put it prior to the equinox in many years.
Also, the Gregorian, Julian, and Hebrew calendars drift at different rates. The Gregorian drift against the equinoctal year is minuscule; it takes 10,000 years to slip a day. The Julian drifts the fastest, while the Jewish calendar is in the middle. Therefore, the earliest possible date of Passover is drifting away from the true equinox, but not as fast as the earliest day for Julian Easter. Julian Easter always follows Passover (a) because the pattern of Jewish leap years keeps Nisan early enough, and (b) because the Julian dates are even more "wrong" than the Jewish dates. "Wrong" in this case means "picks the second full moon after the equinox, rather than the first." For example, in 2002 the Jewish date was right, but the Julian date was a month late;in 2005 both were late. The Gregorian pascalion nearly always picks the right date; I think there's some slight possibility that it could pick too early a day, but as far as I can tell this hasn't happened. However, the drift of the Jewish calendar has meant that when it picks the wrong date, it follows Gregorian Easter. However, this is OK, according to the rules. There is no rule that says that Easter has to follow Passover.