Engine not cranking

Sir, my car battery has some issues.

I do not have multi-meter.
I checked for low voltage by turning headlamps, room lamps etc on. They give reasonable brightness showing there is enough juice on the battery. And, battery fluid level also maintain at proper level.

The car cranks and the cranking sounds is good and loud enough to confirm the battery has got enough voltage. (Before when voltage is low, the crank sounds much much lower and every lights were much very dimmed.)

Now, surprisingly with noting the above cited conditions, I jump start with another battery, contrarily the car starts easily.

Big question is, why with all lights okay and crank sounds give enough noises, the car does not start?
Why in the same condition, the car starts easily with jump start?

Would be pleased to know if you could provide reasons for the problems. Thanks.