Job numbers are calculated using a variety of occupational systems and labor market reporting systems. If any one of these systems changes, the calculation of numbers can change as well.

Occupational systems:

Dictionary of Occupational Titles (DOT, 1991) - Stable since 1998, when the final updates were released
Standard Occupational Classification (SOC, 2000, 2010, 2018)
Census Occupational Codes (1980, 2000, 2010, 2018)
Occupational Requirements Survey (ORS, based now on SOC 2018) - updated annually in late December

Labor Market Data:

Occupational Employment Statistics (OES, based on SOC, released annually in late March or early April)
Long term Employment Projections (based on the OES groups) - released annually in September
Public Use Microdata Sample (PUMS) - Released annually (about mid-year) for 1 or 1-5 years - Used by SkillTRAN to extract household survey-based data about hours worked by Census occupation
Census Data - Reporting wages by Census occupation and by gender annually - late January of each year
North American Industry Classification System (NAICS) - Updated 1997, 2002, 2007, 2012, 2017 - Replaced the older Standard Industrial Classification
County Business Patterns (CBP) - Reported annually (2 year delay) to describe a census of business activity reported on tax and other administrative records by NAICS and total number of employees
Current Employment Statistics - Reports monthly for the prior month re: total employment by NAICS industry (3 and 4-digit NAICS levels)

So how often do the SkillTRAN Industry Context Job Estimates change?

Any change to any one of these above systems can impact the calculation of the SkillTRAN job number estimates. When some systems change, it may take several years for reported statistics to reflect the use of these newest codes. For example, the 2018 Census code changes were timed to align with the 2018 SOC changes. However, it was not until 2020 that Census data numbers were reported using the 2018 Census Occupational coding. The OES Coding system is completely dependent on the SOC and NAICS coding systems, and began reporting its job numbers using some of the newer SOC codes in 2018-2020, along with the newer 2017 NAICS coding. However, the OES program also began to aggregate more SOC occupational groups together as it began its 3 year migration to the full SOC 2018 system. In 2017, the OES reported data on 810 SOC 2010 groups. But in 2018 - 2020, it reported its employment data for only 790 occupational groups, including some "hybrid groups" where it aggregated multiple SOC 2018 occupations into new hybrid classifications. 

So there are multiple triggers for changes in the job numbers:
1. Coding systems change (e.g. SOC, CENSUS, OES, ORS) - each system follows a different update schedule, except the DOT, which is "stable"
2. OES changes for special, temporary hybrid codes for both SOC and NAICS
3. Changing the current (or projected year) in the SkillTRAN DOT Estimate page - which adjusts the numbers based on long term projections for each related NAICS based on the selected OES occupational group
4. Recalculation of the Full-time/Part-time percentages for a Census occupation (crosswalked to the SOC occupation) based on fresh PUMS data
5. SkillTRAN changes to the DOT-NAICS cross-reference table (based on user suggestions and continued SkillTRAN review of its 120,000+ linkages of DOT to NAICS)

 The GOOD NEWS is that these hybrid OEWS occupational groups will be done in September 2022 with the release of the 2021-2031 Employment Projections with 832 distinct SOC 2018 occupations (of the 848 civilian SOC groups). This should remain stable for the next several years.