When a man makes a promise, it is usually based on the strength of his capacity. He is however fallible and can come short of fulfilling that promise. He is human, limited in resources, wisdom, and in all wise; therefore there is a limitation in terms of fulfillment.

Only the All-Wise God is able and faithful enough to perform all that He has promised. With our God, there is no limitations. Also because of the uncompromising nature of God, we are fully convinced that what He had promised He was also able to perform.

Romans 4:21 NKJV