Columbus' original journals were delivered under seal to Ferdinand and Isabella upon his return to Spain. He did not write a "summary letter" as the above answer suggests, nor did he keep two journals. The original journals were misplaced, however Bartolome de las Casas claimed to "remember" the details of the journals and reconstructed them. De las Casas' versions of the journals suggest that Colombus deceived his crew as to the actual distance travelled:
Sunday, 9 September. Sailed this day nineteen leagues, and determined to count less than the true number, that the crew might not be dismayed if the voyage should prove long. In the night sailed one hundred and twenty miles, at the rate of ten miles an hour, which make thirty leagues. The sailors steered badly, causing the vessels to fall to leeward toward the northeast, for which the Admiral reprimanded them repeatedly.
Monday, 10 September. This day and night sailed sixty leagues, at the rate of ten miles an hour, which are two leagues and a half. Reckoned only forty-eight leagues, that the men might not be terrified if they should be long upon the voyage.
It seems strange that he would record his deceit in a journal to be delivered to his sovereigns; so one must accept the above with a healthy degree of skepticism. Additionally, De las Casas was horrified at Columbus' treatment of the Indians whom the latter met, and therefore was not inclined to show Columbus in a favorable light. An example:
I do not, however, see the necessity of fortifying the place, as the people here are simple in war-like matters, as your Highnesses will see by those seven which I have ordered to be taken and carried to Spain in order to learn our language and return, unless your Highnesses should choose to have them all transported to Castile, or held captive in the island. I could conquer the whole of them with fifty men, and govern them as I pleased.
It was de las Casas' version of Columbus' journal that gave rise to the"Black Legend" of Spanish cruelty to the Indians. Whether true or not, it was used to great effect by the British in later wars with the Spanish.
Columbus' reference to himself as "the Admiral" is not remarkable. It was a common practice in that day for one to refer to himself in third person in journals, etc. As far as "admiral," one of the conditions of his sailing for Spain was that he be given the title of Admiral of the Ocean Sea.
Christopher Columbus (1451-1506) apparently kept two journals on his first voyage, a secret one with the actual calculated distance traveled, and the one he made public to his crew, which under-reported the distance.
Unfortunately, his original journal has been lost, but an abstract was written by Spanish historian Bartolome de las Casas in the 1530's, who had access to the original.
Strangely enough, Columbus refers to himself in his writings in the third person, many times conferring upon himself the title of "Admiral."
On the way home, he also wrote a summary letter of his journal to present to Queen Isabella and King Ferdinand, which is still extant.