The explanation is due to the fact that they are not calculated in the same way and not evaluated for the same periods.
The personal dose is background subtracted, all monthly doses below 75 µSv are rounded to 0 in official reports, following the recommendation (original pdf document only available in German) of the Swiss Federal Commission on Radiation Protection, and the 'month' is calculated on a period that depends on your reading frequency. If you read your personal dosimeter on the 1st of January and then the 20th of February, your month of January will be closed by your first February reading and will count 51 days. If you then read your dosimeter on the 2nd of March, the month of February will only count 10 days.
The operational dose does not have background subtracted, no operations are performed on the dose and the monthly dose is simply the sum of all your doses based on calendar months.
Therefore the doses will never be comparable as they are not calculated in the same way and not accounted during the same time periods.
E.g.: If you accumulate 50 µSv each month, at the end of the year your official dose report will show a dose of 0 µSv ( 50 µSv is below 75 µSv so rounded to 0 µSv) whereas your annual operational dose will be 600 µSv.
Additionally, the two dosimeters do not have the same energy response and are not placed in the same place on your body and therefore do not necessarily 'see' the same radiation levels.