It turns out numbers aren't the only things that matter for companies trying to gain entry into the index of Japan's best companies.

Tokyo Electric Power Co. missed out on a place in the government-backed JPX-Nikkei Index 400 in a reshuffle of the gauge's members last week, defying forecasts from at least four brokerages that the utility would gain entry because it meets the index's quantitative criteria. Tepco, as the company is known, tumbled 7.9 percent to ¥815 as of 12:45 p.m. in Tokyo on Monday, on course for its steepest drop since August 2013.

The operator of the crippled Fukushima No. 1 nuclear power plant became eligible for the index after a surge in its earnings and shares, according to brokerages including Nomura Holdings Inc. and Daiwa Securities Group Inc. While the exclusion of Tepco is understandable "on an emotional level," it calls into question the transparency of the selection criteria for an index designed to wring better shareholder returns from Japanese companies, said SMBC Nikko Securities Inc.