It is intentional, since it makes the Jewish calendar be able to be calculated on a regular human not overly complicated cycle.
What’s so complicated about skipping a day or so every hundred years? That’s what we do in the Gregorian Calendar (years evenly divisible by 100 are NOT leap years unless they are also evenly divisible by 400).
We currently celebrate Pesach (and all other holidays) about eight or nine days later in the year than Hillel did… and the problem continues to compound at the rate of a few minutes every year.
The astronomers could not calculate such accurate calendars even in Hillel’s time.
To determine the length of the lunar cycle does not require advanced instruments. Once you realize that a solar eclipse can only occur at the time of the conjunction of the sun and the moon, you can simply count the days between eclipses and dividing by the number of lunations. The more data points you have (i.e. the more observed eclipses), the closer you can arrive at the the true value of the lunar month. You can also use lunar eclipses (since they can only occur when the sun and moon are in opposition). Once you have enough data points, you can arrive at the length of the lunar month to several decimal places. No need for advanced clocks, telescopes or anything else. All you need is the ability to see the sun and/or moon and the ability to count days.
The Wolf