Huggy,
Great question.

When the topic of drill pay and basic pay are discussed in military units between the active and reserve/Guard service members, confusion is not uncommon regarding the differences of basic pay for the active component and drill period pay for the reserve/guard component.
The required understanding is this:
1. Active component members are paid 365 days a year regardless of work performed.
2. Reservists/Guard component members are only compensated for work/duty completed.
3. If Reserve/Guard members are placed on active duty they become eligible for the same pay as active duty counterparts.
Active component examples
30 days vacation earned annually
• Weekends free*
• National holidays*
• Sick days as needed
*As with any job, missions and training may require work on weekends or holidays.
Reserve Component examples: The hours of work and even sometimes specifically what kind of work are specifically defined to earn the drill pay.
Using two 0-4’s. Same 10 years of service. Using the 2016 DFAS pay charts.
Active Component 0-4 over 10 Basic Pay: $6745.80/month
Reserve Component 0-4 over 10 One drill period pay: $224.86/drill
Daily Rate:
Active component member is paid a basic pay of $221.78/day. However, they do receive paid days off unlike the Reserve/Guard member. Adding 30 days leave plus 10 Federal Holidays and using weekends off (104 days) then the active component member receives 365-30-10-104 = 221 days of "paid" work. This equates to $366.29 of pay per day Basic Pay for a day “worked”
Reserve Component is required to perform duty/work to be paid. To be paid a drill period of pay, one must perform a minimum of 4 hours of work. (Can be extended to 24 hours for one drill period). A Reserve/Guard component member could receive 2 drills if worked 8 or more hours for a total of $449.72
What accounts for the disparity of both working a real 8 hour day?
Active component gets many other financial benefits and services from active service not provided to a Guard/Reserve that is effectively paid through the drill pay differential to remain fairly equal in benefit to the service member.
DOD works to set an equivalent financial benefit. Think it is a pretty effective solution.
Cheers