In the climate section of Wikivoyage's guide to Vietnam, it's claimed that the highlands of Vietnam have both colder winters, and hotter summers:
The north has four distinct seasons, with a comparatively chilly winter (temperatures can dip below 15°C/59°F in Hanoi), a hot and wet summer and pleasant spring (Mar-Apr) and autumn (Oct-Dec) seasons. However, in the Highlands both extremes are amplified, with occasional snow in the winter and temperatures hitting 40°C (104°F) in the summer.
Similar things are said here
There are extremes in the Highlands as snow sometimes falls in the winter whilst the summer can see temperatures up around 40ºC.
and a similar thing is said in Wikipedia's article on the geography of Vietnam
Seasonal divisions are more clearly marked in the northern half than in the southern half of the country, where, except in some of the highlands, seasonal temperatures vary only a few degrees, usually in the 21–28 °C (69.8–82.4 °F) range.
I thought that increased altitude merely meant a consistent reduction in temperature.
Does Vietnam's highlands have hotter summers as well as colder winters compared to lower lying areas, and if so, why?