Trying to figure out how key fragments drops work

So, theoretically we're supposed to get 12 key fragments every 4 weeks just by playing the game and staying honorable. I play daily and I'm honor level 4, but I haven't received a single key fragment since 4/25 and I don't understand why.

I used to have only 1 unopened free chest in my loot, but now there are 3. I noticed this trend a few months ago, so I've started recording when and how many key fragments I get from the system.

Here are the results:

• 2/24: 2 (honor 3 checkpoint 1)
• 3/2: 1
• 3/4: 1
• 3/10: 2
• 3/14: 2 (honor 3 checkpoint 2)
• 3/17: 1
• 3/19: 1
• 3/22: 1
• 3/24: 1
• 3/25: 1
• 3/28: 1
• 3/20: 1
• 4/1: 2 (honor 3 checkpoint 3)
• 4/7: 1
• 4/8: 1
• 4/11: 1
• 4/12: 1
• 4/15: 1
• 4/17: 1
• 4/19: 1 + 4 (honor 4 checkpoint 0)
• 4/24: 1
• 4/25: 1

Since 2/24, 70 days have passed, which means 2.5 28-days-long intervals, which should amount to 30 key fragments. If we only count the random drops, the number of key fragments I've received is 20, but if we add the ones I got from honor checkpoints the total amount is 30.

Does that mean that the key fragments we receive from honor checkpoints aren't a bonus rewards, but just an integral part of the system presented in a different way? If that's the case, are we supposed to spend RP to open the free hextech chest that we got from one of the April Foods event missions?