All the plants you planted are cultivars of Lavandin (Lavandula x intermedia), an interspecific hybrid between L. angustifolia (English lavender) and L. latifolia (Portuguese lavender).
Lavandin was bred to have larger flowers and especially yield more essential oil than both parents but it's not a particularly long-lived plant: commercially grown plants in France (Grosso being the most common) are replaced every 4-5 years since they start declining and essential oil yields start dropping.
Lavender plants don't just die like that: they start declining a bit every year until they usually become so unsightly you have no choice but to pull them from the ground.
The longest lived lavender is English lavender (L. angustifolia), which incidentally also supplies the highest quality essential oil. They will live a decade or more even in unfavorable weather but usually require harder pruning than other lavenders. 'Hidcote Blue' is the most common cultivar but I strongly suggest you look into two brand new cultivars 'Havanna Dark' and 'Forever Blue' since they have been bred for longevity and long flowering time. 'Richard Gray' is an exotic possibility if you like true silver-gray foliage.
