That make sense because there is no way to BTC is going down in the investment as compared to the altcoins. Weightage of the of the bitcoin is going to be always higher as compared to the altcoins because its popularity is way higher than them plus people find it very appealing to put their funds into the bitcoin whenever there is mega drop in values like the current one. Meaning they will go and buy more BTC in the down trend and also they will convert their alts coins into the BTC and thus ultimately will be pushing the BTC over the alts. The dominance is going to be higher for this reason.