In 4 weeks they're significant! Running a 'low' and 'mid' km 15s faster is heaps - given you presumably were judging by effort rather than pace. Especially given you're not exactly (a) following a training program to run Fast 1km efforts, nor (b) had any time to make any significant difference. If (for example) you did a 2km time trial you'd probably find a decent improvement.
I like the 3 x 1km S-M-F test set - and used to use it regularly with my coaching group - but it takes some decent intestinal fortitude and near-identical weather conditions for HR to behave similarly each time, let alone your work situation.
Yeah fair point re HR and I know it was decent gains just hoped for more - been slack on diet and sessions so hence a plateau and gains not as good as should be.