I am very new to Japanese, but I noticed this today.
In Wiktionary article on 一歩 we see that the pronounciation is いっぽ. In article for 一 we see that there is a nanori reading いっ. On the other hand in the article for 歩 we see that only sensible reading here would be ほ. But put together we get いっぽ. Also in wikipedia article for っ, it says that sokuon won't appear before h, except in loanwords or nonstandard speech.
So, why there appears to be a っ before h here? And why does it appear to transform the ほ to ぽ?