I agree that it is a surprise that these appointments were announced now rather than on St Georges day-but then recent appointments to the Thistle have departed from being announced on St Andrews day and were instead announced in the New Year honours.
The new appointments come from categories of persons from whom previous garter knights have been named-Lord Lieutenants(present member who are or have been Lord Lieutenants are Sir Timothy Colman,Lord Morris of Aberavon,Sit Thomas Dunne,Lord Shuttleworth,Sir David Brewer and Lady Mary Fagan) and Leaders of the House of Lords(previous garter appointments who were leaders of the House of Lords in the 20th century included Lord Carrington,Lord Hailsham,the 5th Marquess of Salisbury,Viscount Addison,the 7th Earl Stanhope,the 1st Earl of Halifax,the 7th Marquess of Londonderry,the 4th Marquess of Salisbury,the 1st Marquess Curzon,the 1st Marquess of Crewe,the 1st Marquess of Ripon,the 5th Marquess of Lansdowne,the 8th Duke of Devonshire and the 3rd Marquess of Salisbury).
Lord Salisbury is the 9th member of his family to be so honoured(the 1st,2nd & 3rd Earls,and all Marquesses except the 6th).That is only equalled or exceeded by the Howard Dukes of Norfolk(9) and the Cavendish Dukes of Devonshire(11).
The appointments are replacements for Sir Ninian Stephen(d2017) and Sir William Gladstone(d 29 March 2018).No replacement has yet been appointed for Lord Carrington whose death was on 9 July 2018.