Why 112 is pronounced as ippyakujuuni instead of ichihyakujuuni?

Question

I ran across Jim Breen's page (http://nihongo.monash.edu/jnumbers.html) regarding numbers, it links to an image (provided below) which I found confusing.

It reads 112 as ippyakujuuni, and 1386 as sensanbyakuhachijuuroku. I would expect ichihyaku or hyaku, not ippyaku, for 100. And sanhyaku, not sanbyaku, for 300. What is behind this strange reading of 100 there?

There is also similarly strange reading of number 2 036 521 801 in Wikipedia, which is ni-jū oku san-zen rop-pyaku go-jū ni-man sen hap-pyaku ichi. Why san-zen, not san-sen? Why rop-pyaku, not roku-hyaku? Why hap-pyaku, not hachi-hyaku?

Answer 1

• Normally, 112 is read ひゃくじゅうに. いっぴゃく is used in some rare situations, but it's simply wrong elsewhere. See my answer here: In conversation can you just say "万円です" or must you say "一万円です"?
• 300 is always さんびゃく (sanbyaku), not さんひゃく. 3000 is always さんぜん (sanzen), not さんせん. They may seem irregular, but you have to remember them. See this chart.

Answer 2

Many languages tend to simplify or change words to sound better when they are regularly used together in conjunction to form a new 'compound word'. Old English 'Two + ty' became 'twenty'. Italian 'Venti + uno' is pronounced 'ventuno', etc.

Recommend reading up on Japanese phonology (especially rendaku and gemination), and here for answers to a similar questions.