May be due to compression technology,
New box in HEVC H. 264 in sd and H.265 in HD channels.
If the television is HD ready then u will get some sharp picture in sd channels too.
But if it's FullHD then .
{Off topic---if we already have HD TV and HD channels then why need sd channels !
Start HD box with HD channels only NO NEED any SD channel,
All DTH are just showing channels numbers in their bouquet.
In these days lot of streaming application and smart television are available in the market, in which we can watch our favorite movies/tv shows in our time. "without any break".}