Speculation
We can speculate as to some of the reasons for variances. A primary reason could be that some of these astro-calendars are produced by print shops who have no/little astrological expertise and take the information from an ephemeris. There are two primary category of ephemerides -- those calculated for noon of each day, and those calculated for mid-night of each day based on GMT. Within these two groupings, each publisher may or may not include any depth and range of aspects and associated data relating to the planetary positions listed. 12 hours of difference can account for some aspects being closer or further from exactness -- making them non-aspects in some cases.
The printed page, size-wise and font-wise may permit or exclude a desireable amount of information to be included on the page of the ephemeris.
The same applies to the calendar as its size and format can make it easier or more difficult to include a full aspect listing for each day.
For those who do understand astrology and produce a carefully planned astro-calendar, there still can be many differences. If declinations and moon phases are included, then the number of of planetary aspects may have to be reduced due to space.
If the intent is to also offer brief commentary for each day, then even a few aspects may require too much space. In these cases, the aspects printed may be chosen because they support the commentary. Astro-calendars that serve gardening, fishing or romance (for example) can all fall into this problem area.
Those are the primary issues that come to mind. It is also possible that a calendar printed in Europe or Asia and then distributed in the US would have a distinct difference due to time zones and might include/exclude aspects based on when they occured in that original locality.
Hope this helps. Dave
