The first time I took it was in 2014. I scored an 8 on all sections except for listening module on which I scored an 8.5. I took the exam a second time and scored a 9 on all the modules and a mere 6.5 on the writing module.
I read other testimonials online which stated that IELTS tries to keep your score low by following a dubious marking scheme in which they award low scores for the writing section if their other scores are too high. A lot of people have suffered the same fate but have achieved near perfect scores on the PTE or the TOEFL. I am almost certain that I did way better on the writing module the second time.

That would compress the distribution of test scores, and that would not be in the financial interest of the exam provider, nor would it help universities to filter students.
Speaking as someone who has marked and moderated undergraduate essays I do feel that almost everyone has a lower capacity for writing than for the other three parts of language capacity. In particular, the ability of most students to do comparative or critical academic writing is low.

I really doubt they could get away with something like this.

I have taken IELTS (general), TOEFL and TOEIC. I think if you want to make money then you'll have to become the game, cannot be a player. So regardless of your actual English proficiency, they'll measure your testable English proficiency.

I scored 105/120, and 100/120 in TOEFL without any prep in 2009 and 2012 respectively. Does it mean that my English proficiency decreased? or did it spiked again in 2017 when i got a band 8.5 overall out of 9 in IELTS?

I find TOEFL to be highly unnatural. Think of a topic to speak for 60 seconds in the next 45 seconds. and then when i started speaking - heck everyone in that test center was speaking at the same time. couldn't focus and mic was picking up all that crap. TOEFL iBT is computer based, so someone who was fast reached the speaking section before others. When they spoke out loud into their mic, it interrupted others who were maybe still in listening section.

IELTS (general) was much better experience for me. Every person in the room hops through the sections at the same time. No interference. For the Speaking section, you actually have a real conversation with a human being - which is much more natural. Now here is the tricky part. I took my IELTS in Virginia, USA. The teacher/inspector? there was exposed and used to multiple cultures and pronunciation across the globe. So I scored high marks in speaking 8.5/9. While at the same time, one of my dear friend took IELTS in India. Possible the inspectors were only exposed to British pronunciation and it was a gold standard for them. So if someone uttered skedule instead of schedule that's minus 0.5
Some of my friends who scored poorly in the speaking section in India scored highly in US.
Also, IELTS writing penalize spelling mistakes heavily. While most of us are used to type and then right click to spell correct, on paper of course those mistakes do happen. I was extra careful since I have a tendency to make butchery of spelling and grammar. I was so scared of losing points that i swapped words like "necessary" with "required" since was not sure of number of s & c in necessary. Additionally, i followed the simple rule of 3 paragraphs. Opening, argument and Concluding. many people tend to write long complicated theories. Your subject matter is not under the scanner, English is. I kept it very simple as if a 10 year old writes to Santa, made no spelling error - that i can think of - finished the section 15 mins before time and got 8.5

Of course as long as there is a human element in marking it's not fully standardized.

Possible the inspectors were only exposed to British pronunciation and it was a gold standard for them. So if someone uttered skedule instead of schedule that's minus 0.5