It's a reflection of the BLACKCAPS steady progress in Tests - in the last 12 months the team has lost a Test series to England 0-2 (away), drawn with Bangladesh 0-0 (away), beaten the West Indies 2-0 (home) and beaten India 1-0 (home).
In that time the team has developed a consistency of selection, often naming unchanged sides between Tests, formed a formidable seam team of Tim Southee, Trent Boult and Neil Wagner, introduced all-rounders Corey Anderson and Jimmy Neesham to Test match cricket as well as new spin bowler Ish Sodhi. The batsmen hit a rich vein of form in the home summer in particular, with Brendon McCullum's triple and double centuries, and Ross Taylor's three centuries in three Tests among the standouts.
Progress is also reflected in the individual player rankings - Ross Taylor is the number six Test batsman in the world, while Trent Boult and Tim Southee sit at seven and eighth place respectively for bowlers.
Consistent form from senior players like Ross Taylor and Tim Southee have helped the BLACKCAPS rise to sixth in the world rankings.
If it wasn't already, the upcoming three Test series v the West Indies becomes must-win to continue the BLACKCAPS progress. The Windies sit in eighth spot on 76 points, and a series loss will halt the BLACKCAPS upward movement and potentially send them backwards. Likewise, the late-in-the-year Tests v Pakistan, who sit fourth, will be a ripe opportunity to move up.
How do the rankings work?
It's complicated. Rankings are based on your opponent's ranking - from the ICC's Test rankings FAQ: "A rating is worked out by dividing the points scored by the match/series total, with the answer given to the nearest whole number. It can be compared with a batting average but with points instead of total runs scored and a match/series total instead of number of times dismissed." You're best to read the full FAQ yourself for the run down, but (so it seems), it can be quick to fall down the rankings but take longer to go up.
Reliance ICC Test rankings
5. India - 23 matches, 2343 points, 102 rating
6. New Zealand - 26 matches, 2383 points, 92 rating
7. Sri Lanka - 22 matches, 1986 points, 90 rating