Trend analysis of Rainfall

I has rainfall data for which I did linear trend analysis using parametric test for various seasons.

In one season for example, I got slope 6.1 mm/decade and p value 0.0001. while for the other seasons I got slope 4.5 mm/decade and p value=.90. slope indicate that both the data has the trend. One is statistically significant and other not.

My question is, why I am saying to one has trend and other has not? And when both has such high slope why one giving significance result and other not??