Each of these stamps gives you the ability to stay in the country for up to 6 months from the date you entered. The key word here is "STAY". Once you leave the country, your entry is over, and you have no right to stay in the country any further (which isn't an issue, because you've already left!)
Thus your "6 April" stamp was valid from the 6th of April, up until whatever date you next left the UK. At that point, your time is up - even though you were in the country for a lot less than 6 months. At this time, that stamp serves little purpose other than as a historical record of your entry.
The "15 May" stamp is valid from the 15th of May until EITHER 6 months after that date, or until you leave the UK again, whichever occurs first.
So given these pair of stamps, you are allowed stay into the UK until around the middle of November, 2019.