Obviously, the wide range of McConnell's estimated wealth is due to the way the disclosure forms allow a pretty wide range on reports of assets.
His top asset is his holdings in the Vanguard Tax Exempt Money Market, estimated at between $5,001,002 and $25,015,000. His next most valuable asset is his D.C. home (and carriage house rental) which is reported as worth between $1M and $5M.
The rest are investments in a wide variety of stock and bond mutual funds.
Let's look at his Vanguard investments.
Back in 2006, his major Vanguard investment was in its 500 Index Fund, estimated at between $650,003 and $1,350,000.
The next year, McConnell's wealth in that fund jumped to an estimated $1,100,002 to $5,250,000. Then in 2008 (not a good year in the market), McConnell's wealth in that fund dropped a bit (estimated from $500K to $1M), but all of a sudden he had $5,001,002 to $25,015,000 in the Vanguard Tax Exempt Money Market.
The year before, he had a pittance ($1K to $15K) in that fund.
Somehow, some way, McConnell got much richer in 2008, getting at least $5M from somebody.
Jesse Benton, campaign manager for Senate Minority Leader Mitch McConnell (R-Ky.), is resigning his position, the Lexington Herald-Leader reported Friday. The announcement comes as the McConnell campaign wrestles with Benton's possible ties to a bribery scandal when he was political director for then-Rep. Ron Paul (R-Texas) during the 2012 presidential election.
As Paul's political director in 2012, Benton has been tied to a scandal that saw thousands of dollars exchanged for political endorsements prior to the Iowa caucuses. On Wednesday, former Iowa state Sen. Kent Sorenson pleaded guilty to charges related to receiving under-the-table payments for switching his support from U.S. Rep. Michele Bachmann (R-Minn.) to Paul. Sorenson then lied to investigators about the payments, according to the Department of Justice. Sorenson's guilty plea was accompanied by two documents that could potentially implicate Benton, the Herald-Leader notes.

Fat Bastardo's Op Ed: 
How can a US senator accumulate more tens of millions of dollars without massive criminal activity? Answer, THEY CAN'T!

Mitch McConnell is a contemptible scumbag.  The idea that his wealth is solely the result of an inheritance is total BS. He got wealthy the way other Republicans get wealthy, through bribes, kickbacks and insider trading.