[Strong or weak day master?]

Hi Master Cecil and Robert,

My Ba Zi is as follows (9th May 1979, 8:30am):

Year (Stem/Branch): Ji / Wei
Month (Stem/Branch): Ji / Si
Day (Stem/Branch): Bing / Zi
Hour (Stem/Branch): Ren / Chen

Accordingly, my day master is Bing and since I am born in the summer season (Si), my chart is naturally hot and dry and my day master should be a strong fire day master. Water in this case will be a favorable element.

However, using the online ba zi calculator provided on this site, it determined that I am a weak fire day master instead, and that water is my most unfavorable element.

Could you kindly explain why this is the case?
Thank you very much for your assistance.