Despite both Qt and KDE having their major version number the same, they are updated on very different schedules which have no particular relation to each-other.
Qt has a much broader scope of packages of all types depending on it than KDE, which is exclusively desktop oriented.
As of KDE-5.10, Qt 5.7 or later is required ,as of KDE-5.12, Qt 5.9 or later, and as of KDE-5.13, it needs Qt 5.10 or later
|Version||Release Type||Release Date|
|5.12.0||LTS Release||Thu 2018-02-01|
|Version||Release Date||Support Until|
|5.7||16 June 2016||16.06.2017|
|5.8||23 January 2017^^||23.01.2018|
|5.9 LTS||31 May 2017^^||31.05.2020^^|
|5.10||30 November 2017||30.10.2018|
|5.11||22 May 2018||2019|
As of KDE-5.10 (2017-05-25), Qt 5.7 (2016-06-16) or later is required (~1yr); as of KDE-5.12 (2018-02-01), Qt 5.9 (2017-05-31) or later (~8mo); and as of KDE-5.13 (2018-06-07), it needs Qt 5.10 (2017-11-30) or later (~7mo).
Following this trend, we're likely looking at nearly another year before Qt 5.11 becomes the minimum version, and in the mean time have other non-KDE packages that can't build or don't work properly because we don't provide newer Qt versions until kde requires them.