Expand Support Gem has inconsistencies for reducing the time to gain a seal when scaling cast speed

The support gem 'Expand' has the following line:

"

Supported Spells gain a Seal every 200% of cast time, to a maximum of 3 Seals

In my testing, I have seen that scaling cast time to get the seals faster do not seem to gain seals much faster. I removed my cast speed gear and did 10 stopwatch tests with some cast speed still remaining in the tree, and saw that the average time to gain a seal was approximately 5-6 seconds, which falls in line with my cast speed (0.73 casts/second) from the tooltip.

When putting on cast speed gear, I got the number down to 0.53 casts/second, which means my expected time to get all 3 seals is 0.53*2*3 = 3.18 seconds.

However, what I observed after 10 tests is an average of about 4.5-5.5 seconds to get all 3 seals back after casting.

In addition, I noticed that there was some inconsistencies with the timing of the first seal that is gained. Sometimes it is slow (2.5s) and sometimes it is fast (0.78s), and I don't know why it is being inconsistent here.

Please let me know if there is any other additional information I can provide.